mmx: Improve detection of support for "K" constraint

Older versions of clang emitted an error on the "K" constraint, but at
least since version 3.7 it is supported. Just like gcc, this
constraint is only allowed for constants, but apparently clang
requires them to be known before inlining.

Using the macro definition _mm_shuffle_pi16(A, N) ensures that the "K"
constraint is always applied to a literal constant, independently from
the compiler optimizations and allows building pixman-mmx on modern
clang.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Andrea Canciani <ranma42@gmail.com>