[X86] Define LAHF_SAHF if feature 'sahf' is set or 32-bit mode
GCC 11 will define this macro.
In LLVM, the feature flag only applies to 64-bit mode and we always define the
macro in 32-bit mode. This is different from GCC -m32 in which -mno-sahf can
suppress the macro. The discrepancy can unlikely cause trouble.
Reviewed By: craig.topper
Differential Revision: https://reviews.llvm.org/D89198