tests/tcg: i386: add MMX and 3DNow! tests

tests/tcg: i386: add MMX and 3DNow! tests

Adjust the test-avx.py generator to produce tests specifically for
MMX and 3DNow.  Using a separate generator introduces some code
duplication, but is a simpler approach because of test-avx's extra
complexity to support 3- and 4-operand AVX instructions.

If needed, a common library can be introduced later.

While at it, for consistency move all the -cpu max rules to the
same place.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>