X86: Don't make illegal GOTTPOFF relocations

"ELF Handling for Thread-Local Storage" specifies that R_X86_64_GOTTPOFF
relocation target a movq or addq instruction.

Prohibit the truncation of such loads to movl or addl.

This fixes PR22083.

Differential Revision: http://reviews.llvm.org/D6839

llvm-svn: 225250