skein_block_asm.S: use #ifdef not .ifdef, for Clang IAS

Clang IAS does not support the --defsym argument, and

.ifndef SKEIN_USE_ASM

gets turned into

.ifndef 1792

by the preprocessor, which results in

error: expected identifier after '.ifdef'
.ifndef 1792

^

Use #ifdef instead, which still works with GNU as.

Reviewed by: cem
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D25154