[X86] Fix some signedness errors in x86 headers

Another step toward enabling full -Wsystem-headers testing across all x86 headers

Fix a number of cases where the arg / return value signedness doesn't match the C/C++ intrinsic.

So far I've just added explicit casts as necessary, but we might want to address some of the mismatches directly

Differential Revision: https://reviews.llvm.org/D125164