mmx: Fix _mm_empty problems for over_8888_8888/over_8888_n_8888

Using "--disable-sse2 --disable-ssse3" configure options and
CFLAGS="-m32 -O2 -g" on an x86 system results in pixman "make check"
failures:

    ../test-driver: line 95: 29874 Aborted
    FAIL: affine-test
    ../test-driver: line 95: 29887 Aborted
    FAIL: scaling-test

One _mm_empty () was missing and another one is needed to workaround
an old GCC bug https://gcc.gnu.org/PR47759 (GCC may move MMX instructions
around and cause test suite failures).

Reviewed-by: Matt Turner <mattst88@gmail.com>