Allow assembling skein_block_asm.s with clang

GNU as seems to allow macro arguments without the '\' but clang is more
strict in that regard.
This change makes the source code compatible with LLVM's but does not yet
change the build system or rename it to .S.

The new code assembles identically with GNU as 2.17.50.

Reviewed By: emaste
Differential Revision: https://reviews.freebsd.org/D25143