PowerPC TPREL16_HA/LO reloc optimization

In the TLS GD/LD to LE optimization, ld replaces a sequence like

 addi 3,2,x@got@tlsgd		R_PPC64_GOT_TLSGD16	x
 bl __tls_get_addr(x@tlsgd)	R_PPC64_TLSGD		x
				R_PPC64_REL24		__tls_get_addr
 nop

with

 addis 3,13,x@tprel@ha		R_PPC64_TPREL16_HA	x
 addi 3,3,x@tprel@l		R_PPC64_TPREL16_LO	x
 nop

When the tprel offset is small, this can be further optimized to

 nop
 addi 3,13,x@tprel
 nop

bfd/
	* elf64-ppc.c (struct ppc_link_hash_table): Add do_tls_opt.
	(ppc64_elf_tls_optimize): Set it.
	(ppc64_elf_relocate_section): Nop addis on TPREL16_HA, and convert
	insn on TPREL16_LO and TPREL16_LO_DS relocs to use r13 when
	addis would add zero.
	* elf32-ppc.c (struct ppc_elf_link_hash_table): Add do_tls_opt.
	(ppc_elf_tls_optimize): Set it.
	(ppc_elf_relocate_section): Nop addis on TPREL16_HA, and convert
	insn on TPREL16_LO relocs to use r2 when addis would add zero.
gold/
	* powerpc.cc (Target_powerpc::Relocate::relocate): Nop addis on
	TPREL16_HA, and convert insn on TPREL16_LO and TPREL16_LO_DS
	relocs to use r2/r13 when addis would add zero.
ld/
	* testsuite/ld-powerpc/tls.s: Add calls with tls markers.
	* testsuite/ld-powerpc/tls32.s: Likewise.
	* testsuite/ld-powerpc/powerpc.exp: Run tls marker tests.
	* testsuite/ld-powerpc/tls.d: Adjust for TPREL16_HA/LO optimization.
	* testsuite/ld-powerpc/tlsexe.d: Likewise.
	* testsuite/ld-powerpc/tlsexetoc.d: Likewise.
	* testsuite/ld-powerpc/tlsld.d: Likewise.
	* testsuite/ld-powerpc/tlsmark.d: Likewise.
	* testsuite/ld-powerpc/tlsopt4.d: Likewise.
	* testsuite/ld-powerpc/tlstoc.d: Likewise.