Properly handle 64-bit GOT relocations

This patch fixes 2 issues:

1. Since the GOT offset is always negative, we need to use signed int
to support 64-bit GOT relocations.
2. R_X86_64_PLTOFF64 uses the address of GLOBAL_OFFSET_TABLE, which is
the address of the .got.plt section, not the .got section.

	PR gold/16945
	* x86_64.cc (Target_x86_64::Relocate::relocate): Use signed int
	for got_offset.  Properly get GOT address for R_X86_64_PLTOFF64.
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