[GOLD] PowerPC64 pc-relative TLS support

[GOLD] PowerPC64 pc-relative TLS support

Gold version of git commit c213164ad2.

elfcpp/
	* powerpc.h (R_PPC64_TPREL34, R_PPC64_DTPREL34),
	(R_PPC64_GOT_TLSGD34, R_PPC64_GOT_TLSLD34),
	(R_PPC64_GOT_TPREL34, R_PPC64_GOT_DTPREL34): Define.
gold/
	* powerpc.cc (Target_powerpc::Scan::get_reference_flags): Set
	flags for new relocations, and some missing older relocs.
	(Target_powerpc::Scan::local): Handle new pcrel tls relocs.
	Call set_has_static_tls for tprel relocs.
	(Target_powerpc::Scan::global): Likewise.
	(Target_powerpc::Relocate::relocate): Handle new pcrel tls relocs.