[X86] Avoid %fs:(%eax) references in x32 mode

The ABI explains that %fs:(%eax) zero-extends %eax to 64 bits, and adds
that the TLS base address, but that the TLS base address need not be
at the start of the TLS block, TLS references may use negative offsets.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D93158