[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