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>
