PowerPC64 tls_get_addr_desc static support

PowerPC64 tls_get_addr_desc static support

This provides a linker generated __tls_get_addr_desc wrapper function
preserving registers around a __tls_get_addr call.  The idea being to
support __tls_get_addr_desc without requiring a glibc update.

bfd/
	* elf64-ppc.c (struct ppc_link_hash_table): Add tga_group.
	(ppc64_elf_archive_symbol_lookup): Extract __tls_get_addr_opt for
	__tls_get_addr_desc.
	(ppc64_elf_size_stubs): Add section for linker generated
	__tls_get_addr_desc wrapper function.  Loop at least once if
	generating this function.
	(emit_tga_desc, emit_tga_desc_eh_frame): New functions.
	(ppc64_elf_build_stubs): Generate __tls_get_addr_desc.
ld/
	* testsuite/ld-powerpc/tlsdesc3.d,
	* testsuite/ld-powerpc/tlsdesc3.wf,
	* testsuite/ld-powerpc/tlsdesc4.d,
	* testsuite/ld-powerpc/tlsdesc4.s,
	* testsuite/ld-powerpc/tlsdesc4.wf: New tests.
	* testsuite/ld-powerpc/powerpc.exp: Run them.