[GOLD] R_PPC64_REL16_HIGH relocs

[GOLD] R_PPC64_REL16_HIGH relocs

These relocs have been around for quite a while.  It's past time gold
supported them.

elfcpp/
	* powerpc.h (R_PPC64_REL16_HIGH, R_PPC64_REL16_HIGHA),
	(R_PPC64_REL16_HIGHER, R_PPC64_REL16_HIGHERA),
	(R_PPC64_REL16_HIGHEST, R_PPC64_REL16_HIGHESTA): Define.

gold/
	* powerpc.cc (Target_powerpc::Scan::get_reference_flags): Handle
	REL16_HIGH* relocs.
	(Target_powerpc::Scan::local): Likewise.
	(Target_powerpc::Scan::global): Likewise.
	(Target_powerpc::Relocate::relocate): Likewise.