gcc.dg/pr44194-1.c: Skip for mmix.

gcc.dg/pr44194-1.c: Skip for mmix.

The test makes sense only for targets that return the
"struct { int a, b, c; }" in registers (not in memory).

Starting a skip-construct is IMHO better than another iteration of
that obscuring "{ ... && { !  mytarget-*-* } }".  New targets can just
append to the list without additional {}:s.  I chose not to "convert"
any of the previous exclusions, as without targets to test, I'd surely
mess up {}-pairs.

A new effective_target would be even better, but such a
check_effective_target_returns_struct_in_memory (or complementary,
_in_registers) would surely have to be parametrized on the size and
type of the returned blob.

Maybe best to restrict to just x86_64, as seems to have been the
original problem target.

gcc/testsuite:
	* gcc.dg/pr44194-1.c: Skip for mmix.