add a note about using clang for Apple systems
add a note about using clang for Apple systems

After a long investigation, it seems like the GCC provided by Apple has
a bug when generating SIMD code from compiler intrinsics.

It happens when some combinations of optimization and inlining occurs:
it converts a `_mm_and_si128` into a `psrld` of 31 bits instead of a
`pand`.

vrancurel/ntl,vrancurel/ntl,vrancurel/ntl