Enable __MINGW_FORCE_SYS_INTRINS for Clang

See https://github.com/msys2/CLANG-packages/issues/6 for the details

The intrin.h header tries to define types like e.g. __m64
if __MMX__ isn't defined. With Clang, mmintrin.h does define such
a type even if __MMX__ isn't defined (and mmintrin.h is included
implicitly via x86intrin.h).

This fixes including intrin.h if building with Clang with -march=i686,
which makes __MMX__ not be defined.

Signed-off-by: Martin Storsjö <martin@martin.st>

Bug: 405855167
Change-Id: I4aa3ec235f95dc780b2f730d7620d1f770b75893