Do not pass `-msse2`, `-mavx2`, etc. flags to the compiler when
compiling `shuffle.c`.  From what I can see, the file itself does not
use any of these intrinsics, and they are only used by functions
declared in `bitshuffle-*.c` and `shuffle-*.c` (where the respective
flags are still passed).  This prevents the compiler from incidentally
optimizing the code called independenlty of the runtime CPU check to
these instruction sets, effectively causing `SIGILL` on other CPUs.

I have verified that this fixes the issue on `-march=znver2`, but also
does not cause any issues on `-march=x86-64` and `-march=i686`.

Fixes Blosc#621

…ation

Add runtime checks for use of SIMD implementations and stubs functions if not available.