Don't build skein_block_asm.s if we don't have an as binary

This fixes building libmd on MacOS/Linux. The real fix is probably to
build it as a .S file with $CC instead. It might also be better to just
compile the C file in userspace since the compiler can the use SSE/AVX.

Reviewed By: emaste, brooks
Approved By: jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D16844