Power10 stub selection

Power10 stub selection

This patch better supports mixing of power10 and non-power10 code,
as might be seen in a cpu-optimized library using ifuncs to select
functions optimized for a given cpu.  Using -Wl,--no-power10-stubs
isn't that good in this situation since non-power10 notoc stubs are
slower and larger than the power10 variants, which you'd like to use
on power10 code paths.

With this change, power10 pc-relative code that makes calls marked
@notoc uses power10 stubs if stubs are necessary, and other calls use
non-power10 instructions in stubs.  This will mean that if gcc is
generating code for -mcpu=power10 but with pc-rel disabled then you'll
get the older stubs even on power10 (unless you force with
-Wl,--power10-stubs).  That shouldn't be too big a problem: stubs that
use r2 are reasonable.  It's just the ones that set up addressing
using "mflr 12; bcl 20,31,.+4; mflr 11; mtlr 12" that should be
avoided if possible.

bfd/
	* elf64-ppc.c (struct ppc_link_hash_table): Add has_power10_relocs.
	(select_alt_stub): New function.
	(ppc_get_stub_entry): Use it here.
	(ppc64_elf_check_relocs): Set had_power10_relocs rather than
	power10_stubs.
	(ppc64_elf_size_stubs): Clear power10_stubs here instead.  Don't
	merge notoc stubs with other varieties when power10_stubs is "auto".
	Instead dup the stub hash table entry.
	(plt_stub_size, ppc_build_one_stub, ppc_size_one_stub): Adjust
	tests of power10_stubs.
ld/
	* emultempl/ppc64elf.em (power10-stubs): Accept optional "auto" arg.
	* ld.texi (power10-stubs): Update.
	* testsuite/ld-powerpc/callstub-1.d: Force --power10-stubs.
	* testsuite/ld-powerpc/callstub-2.d: Relax branch offset comparison.
	* testsuite/ld-powerpc/callstub-4.d: New test.
	* testsuite/ld-powerpc/notoc.d: Force --no-power10-stubs.
	* testsuite/ld-powerpc/notoc3.d,
	* testsuite/ld-powerpc/notoc3.s,
	* testsuite/ld-powerpc/notoc3.wf: New test.
	* testsuite/ld-powerpc/powerpc.exp: Run new tests.  Pass
	--no-power10-stubs for notoc link.

(cherry picked from commit e10a07b32dc1faed25b5bbcbbc47d68e2ff537b9)