[X86] Disable muloti4 libcalls for x86-64.

This library function only exists in compiler-rt not libgcc. So
this would fail to link unless we were linking with compiler-rt.

This is consistent with the recent removal of calls to mulodi4 on
32-bit targets like D108928.

I suppose maybe we could keep the libcalls for platforms like
Darwin that use compiler-rt exclusively?

Reviewed By: nickdesaulniers, MaskRay

Differential Revision: https://reviews.llvm.org/D109385