[ELF][X86_64] Use R_GOTREL_FROM_END instead of R_GOTREL for R_X86_64_GOTOFF64

Summary:
R_X86_64_GOTOFF64: S + A - GOT
R_X86_64_GOTPC{32,64}: GOT + A - P (R_GOTONLY_PC_FROM_END)

R_X86_64_GOTOFF64 should use R_GOTREL_FROM_END so that in conjunction with
R_X86_64_GOTPC{32,64}, the `GOT` term is neutralized. This also matches
the handling of R_386_GOTOFF (S + A - GOT).

Reviewers: ruiu, espindola

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D48095

llvm-svn: 334672