Gold: Don't fail on R_X86_64_[REX_]GOTPCRELX relocations

Gold: Don't fail on R_X86_64_[REX_]GOTPCRELX relocations

This patch updates gold to treat the R_X86_64_GOTPCRELX and
R_X86_64_REX_GOTPCRELX relocations proposed in

https://groups.google.com/forum/#!topic/x86-64-abi/n9AWHogmVY0

the same as R_X86_64_GOTPCREL.  FIXME: Gold should perform the
transformations as suggested.

elfcpp/

	* x86_64.h (R_X86_64_GOTPCRELX): New.
	(R_X86_64_REX_GOTPCRELX): Likewise.

gold/

	* x86_64.cc (Target_x86_64<size>::Scan::get_reference_flags):
	Treat R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX the same
	as R_X86_64_GOTPCREL.
	(Target_x86_64<size>::Scan::local): Likewise.
	(Target_x86_64<size>::Scan::possible_function_pointer_reloc):
	Likewise.
	(Target_x86_64<size>::Scan::global): Likewise.
	(Target_x86_64<size>::Relocate::relocate): Likewise.
	(Target_x86_64<size>::Relocatable_size_for_reloc::get_size_for_reloc):
	Likewise.