[X86] Add -mskip-rax-setup support to align with GCC

AMD64 ABI mandates caller to specify the number of used SSE registers
when passing variable arguments.
GCC also provides option -mskip-rax-setup to skip the setup of rax when
SSE is disabled. This helps to reduce the code size, see pr23258.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D112413