PR 21847, Don't default PowerPC64 to --plt-localentry

PR 21847, Don't default PowerPC64 to --plt-localentry

The big comment in ppc64_elf_tls_setup says why.  I've also added some
code to the bfd linker that catches the -lpthread -lc symbol
differences and disable generation of optimized call stubs even when
--plt-localentry is activated.  Gold doesn't yet have that.

	PR 21847
bfd/
	* elf64-ppc.c (struct ppc_link_hash_entry): Add non_zero_localentry.
	(ppc64_elf_merge_symbol): Set non_zero_localentry.
	(is_elfv2_localentry0): Test non_zero_localentry.
	(ppc64_elf_tls_setup): Default to --no-plt-localentry.
gold/
	* powerpc.cc (Target_powerpc::scan_relocs): Default to
	--no-plt-localentry.
ld/
	* ld.texinfo (plt-localentry): Document.