PowerPC TPREL_HA/LO optimisation

ppc64 ld optimises sequences like the following
	addis 3,13,wot@tprel@ha
	lwz 3,wot@tprel@l(3)
to
	nop
	lwz 3,wot@tprel(13)
when "wot" is located near enough to the thread pointer.
However, the ABI doesn't require that R_PPC64_TPREL16_HA always be on
an addis rt,13,imm instruction, and while ld checked for that on the
high-part instruction it didn't disable the optimisation on the
low-part instruction.  This patch fixes that problem, disabling the
tprel optimisation globally if high-part instructions don't pass
sanity checks.  The optimisation is also enabled for ppc32, where
before ld.bfd had the code in the wrong place and ld.gold had it in a
block only enabled for ppc64.

bfd/
	* elf32-ppc.c (ppc_elf_check_relocs): Set has_tls_reloc for
	high part tprel16 relocs.
	(ppc_elf_tls_optimize): Sanity check high part tprel16 relocs.
	Clear do_tls_opt on odd instructions.
	(ppc_elf_relocate_section): Move TPREL16_HA/LO optimisation later.
	Don't sanity check them here.
	* elf64-ppc.c (ppc64_elf_check_relocs): Set has_tls_reloc for
	high part tprel16 relocs.
	(ppc64_elf_tls_optimize): Sanity check high part tprel16 relocs.
	Clear do_tls_opt on odd instructions.
	(ppc64_elf_relocate_section): Don't sanity check TPREL16_HA.
ld/
	* testsuite/ld-powerpc/tls32.d: Update for TPREL_HA/LO optimisation.
	* testsuite/ld-powerpc/tlsexe32.d: Likewise.
	* testsuite/ld-powerpc/tlsldopt32.d: Likewise.
	* testsuite/ld-powerpc/tlsmark32.d: Likewise.
	* testsuite/ld-powerpc/tlsopt4_32.d: Likewise.
	* testsuite/ld-powerpc/tprel.s,
	* testsuite/ld-powerpc/tprel.d,
	* testsuite/ld-powerpc/tprel32.d: New tests.
	* testsuite/ld-powerpc/tprelbad.s,
	* testsuite/ld-powerpc/tprelbad.d: New test.
	* testsuite/ld-powerpc/powerpc.exp: Run them.
gold/
	* powerpc.cc (Target_powerpc): Add tprel_opt_ and accessors.
	(Target_powerpc::Scan::local): Sanity check tprel high relocs.
	(Target_powerpc::Scan::global): Likewise.
	(Target_powerpc::Relocate::relocate): Control tprel optimisation
	with tprel_opt_ and enable for 32-bit.

(cherry picked from commit 252dcdf4)