[Morello] Add interworking and range extension veneers

[Morello] Add interworking and range extension veneers

Add veneers to branch from A64 to C64 and vice versa and for range
extension from C64 to C64.  The veneers are named as
__foo_a64c64_veneer, __foo_c64a64_veneer or simply __foo_veneer
(where foo is the target function) based on whether the branch is from
A64 to C64, the other way around or for extended range.

A64 to C64 needs an additional BX since the ADRP in the veneer does
not generate a valid capability without the switch using BX.  As a
result, the addendum LSB is no longer important for A64 -> C64 switch,
but we keep it anyway so that we can use the same veneer for long
range C64 to C64 branches.

bfd/ChangeLog:

2020-10-20  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* elfnn-aarch64.c (STUB_ENTRY_NAME): Add format specifier for
	veneer type.
	(C64_MAX_ADRP_IMM, C64_MIN_ADRP_IMM): New macros.
	(aarch64_branch_reloc_p, c64_valid_for_adrp_p,
	aarch64_interwork_stub): New functions.
	(aarch64_c64_branch_stub, c64_aarch64_branch_stub): New stubs.
	(elf_aarch64_stub_type): New members.
	(aarch64_type_of_stub): Support C64 stubs.
	(aarch64_lookup_stub_type_suffix): New function.
	(elfNN_aarch64_stub_name): Use it.
	(elfNN_aarch64_get_stub_entry): Add stub_type argument.
	Adjust callers.  Support C64 stubs.
	(aarch64_build_one_stub): Likewise.
	(aarch64_size_one_stub): Likewise.
	(elfNN_aarch64_size_stubs): Likewise.
	(elfNN_aarch64_build_stubs): Save and return error if stub
	building failed.
	(elfNN_aarch64_final_link_relocate): Emit stubs based on
	whether source and target of a branch are different.
	(aarch64_map_one_stub): Emit mapping symbol for C64 stubs.

ld/ChangeLog:

2020-10-20  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* testsuite/ld-aarch64/aarch64-elf.exp: Add test.
	* testsuite/ld-aarch64/morello-stubs-static.d: New file.
	* testsuite/ld-aarch64/morello-stubs.d: New file.
	* testsuite/ld-aarch64/morello-stubs.ld: New file.
	* testsuite/ld-aarch64/morello-stubs.s: New file.

The jump targets have limited range (i.e. limited by ADRP range) and
hence cannot be used for very long jumps.  The linker will throw an
error for such out of range jumps.