util/u_math: Use xmmintrin.h whenever possible.

util/u_math: Use xmmintrin.h whenever possible.

It seems  __builtin_ia32_ldmxcsr is only available on gcc and only when
-msse is used. xmmintrin.h/pmmintrin.h provide portable intrinsics, but
these too are only available with gcc when -msse/-msse3 are set.

scons build always sets -msse on x86 builds, but autotools doesn't seem
to.

We could try to get this working on gcc x86 without -msse by emitting
assembly, but I believe that in this day and age we really should be
building Mesa with -msse and -msse2.