X86: Use R_X86_64_TPOFF64 for FK_Data_8

Summary:
LLVM would crash when trying to come up with a relocation type for
assembly like:
movabsq $V@TPOFF, %rax

Instead, we say the relocation type is R_X86_64_TPOFF64.

Fixes PR17274.

Reviewers: dblaikie, nrieck, rafael

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1717