[libc] Suppress GCC's <mm_malloc.h> in freestanding builds (#234863)

Unlike Clang, GCC's <xmmintrin.h> (included via <immintrin.h>)
unconditionally includes <mm_malloc.h> even with -ffreestanding, which
in turn includes <stdlib.h>.

In full-build mode, targets that use x86 intrinsics (such as op_x86.h)
don't declare a dependency on libc.include.stdlib (and they don't really
need it). If these targets are built before our stdlib.h header is
generated, this will end up including the system stdlib.h -- and fail
miserably because of mixing of standard libraries. This can happen
already, but the problem gets more acute if we remove (#234150)
transitive full-header dependencies from the proxy headers (as it lets
us build these targets much sooner).

Since we don't use _mm_malloc or _mm_free, I'm suppressing <mm_malloc.h>
by defining its include guard (_MM_MALLOC_H_INCLUDED) on x86/x86_64 GCC
full builds.

Assisted-by: Gemini
GitOrigin-RevId: ffea91bfcff6ba858661698311c66dd8618527ce