BFD: Let targets handle relocations against absolute symbols

BFD: Let targets handle relocations against absolute symbols

Fix a generic BFD issue with relocations against absolute symbols, which
are installed without using any individual relocation handler provided
by the backend.  This causes any absolute section's addend to be lost on
REL targets such as o32 MIPS, and also relocation-specific calculation
adjustments are not made.

As an example assembling this program:

$ cat test.s
	.text
foo:
	b	bar
	b	baz

	.set	bar, 0x1234
$ as -EB -32 -o test-o32.o test.s
$ as -EB -n32 -o test-n32.o test.s

produces this binary code:

$ objdump -dr test-o32.o test-n32.o

test-o32.o:     file format elf32-tradbigmips

Disassembly of section .text:

00000000 <foo>:
   0:	10000000 	b	4 <foo+0x4>
			0: R_MIPS_PC16	*ABS*
   4:	00000000 	nop
   8:	1000ffff 	b	8 <foo+0x8>
			8: R_MIPS_PC16	baz
   c:	00000000 	nop

test-n32.o:     file format elf32-ntradbigmips

Disassembly of section .text:

00000000 <foo>:
   0:	10000000 	b	4 <foo+0x4>
			0: R_MIPS_PC16	*ABS*+0x1230
   4:	00000000 	nop
   8:	10000000 	b	c <foo+0xc>
			8: R_MIPS_PC16	baz-0x4
   c:	00000000 	nop
$

where it is clearly visible in `test-o32.o', which uses REL relocations,
that the absolute section's addend equivalent to the value of `bar' -- a
reference to which cannot be fully resolved at the assembly time,
because the reference is PC-relative -- has been lost, as has been the
relocation-specific adjustment of -4, required to take into account the
PC+4-relative calculation made by hardware with branches and seen in the
external symbol reference to `baz' as the `ffff' addend encoded in the
instruction word.  In `test-n32.o', which uses RELA relocations, the
absolute section's addend has been correctly retained.

Give precedence then in `bfd_perform_relocation' and
`bfd_install_relocation' to any individual relocation handler the
backend selected may have provided, while still resorting to the generic
calculation otherwise.  This retains the semantics which we've had since
forever or before the beginning of our repository history, and is at the
very least compatible with `bfd_elf_generic_reloc' being used as the
handler.

Retain the `bfd_is_und_section' check unchanged at the beginning of
`bfd_perform_relocation' since this does not affect the semantics of the
function.  The check returns the same `bfd_reloc_undefined' code the
check for a null `howto' does, so swapping the two does not matter.
Also the check is is mutually exclusive with the `bfd_is_abs_section'
check, since a section cannot be absolute and undefined both at once, so
swapping the two does not matter either.

With this change applied the program quoted above now has the in-place
addend correctly calculated and installed in the field being relocated:

$ objdump -dr fixed-o32.o

fixed-o32.o:     file format elf32-tradbigmips

Disassembly of section .text:

00000000 <foo>:
   0:	1000048c 	b	1234 <bar>
			0: R_MIPS_PC16	*ABS*
   4:	00000000 	nop
   8:	1000ffff 	b	8 <foo+0x8>
			8: R_MIPS_PC16	baz
   c:	00000000 	nop
$

Add a set of MIPS tests to cover the relevant cases, including absolute
symbols with addends, and verifying that PC-relative relocations against
symbols concerned resolve to the same value in the final link regardless
of whether the REL or the RELA relocation form is used.  Exclude linker
tests though which would overflow the in-place addend on REL targets and
use them as dump patterns for RELA targets only.

	bfd/
	* reloc.c (bfd_perform_relocation): Try the `howto' handler
	first with relocations against absolute symbols.
	(bfd_install_relocation): Likewise.

	gas/
	* testsuite/gas/mips/mips16-branch-absolute.d: Update patterns.
	* testsuite/gas/mips/branch-absolute.d: New test.
	* testsuite/gas/mips/branch-absolute-n32.d: New test.
	* testsuite/gas/mips/branch-absolute-n64.d: New test.
	* testsuite/gas/mips/branch-absolute-addend-n32.d: New test.
	* testsuite/gas/mips/branch-absolute-addend-n64.d: New test.
	* testsuite/gas/mips/mips16-branch-absolute-n32.d: New test.
	* testsuite/gas/mips/mips16-branch-absolute-n64.d: New test.
	* testsuite/gas/mips/mips16-branch-absolute-addend-n32.d: New
	test.
	* testsuite/gas/mips/mips16-branch-absolute-addend-n64.d: New
	test.
	* testsuite/gas/mips/micromips-branch-absolute.d: New test.
	* testsuite/gas/mips/micromips-branch-absolute-n32.d: New test.
	* testsuite/gas/mips/micromips-branch-absolute-n64.d: New test.
	* testsuite/gas/mips/micromips-branch-absolute-addend-n32.d: New
	test.
	* testsuite/gas/mips/micromips-branch-absolute-addend-n64.d: New
	test.
	* testsuite/gas/mips/branch-absolute.s: New test source.
	* testsuite/gas/mips/branch-absolute-addend.s: New test source.
	* testsuite/gas/mips/mips16-branch-absolute-addend.s: New test
	source.
	* testsuite/gas/mips/micromips-branch-absolute.s: New test
	source.
	* testsuite/gas/mips/micromips-branch-absolute-addend.s: New
	test source.
	* testsuite/gas/mips/mips.exp: Run the new tests.

	ld/
	* testsuite/ld-mips-elf/branch-absolute.d: New test.
	* testsuite/ld-mips-elf/branch-absolute-n32.d: New test.
	* testsuite/ld-mips-elf/branch-absolute-n64.d: New test.
	* testsuite/ld-mips-elf/branch-absolute-addend.d: New test.
	* testsuite/ld-mips-elf/branch-absolute-addend-n32.d: New test.
	* testsuite/ld-mips-elf/branch-absolute-addend-n64.d: New test.
	* testsuite/ld-mips-elf/micromips-branch-absolute.d: New test.
	* testsuite/ld-mips-elf/micromips-branch-absolute-n32.d: New
	test.
	* testsuite/ld-mips-elf/micromips-branch-absolute-n64.d: New
	test.
	* testsuite/ld-mips-elf/micromips-branch-absolute-addend.d: New
	test.
	* testsuite/ld-mips-elf/micromips-branch-absolute-addend-n32.d:
	New test.
	* testsuite/ld-mips-elf/micromips-branch-absolute-addend-n64.d:
	New test.
	* testsuite/ld-mips-elf/mips-elf.exp: Run the new tests, except
	from `branch-absolute-addend' and
	`micromips-branch-absolute-addend', referred indirectly only.