[skein] Fix compilation on gnu assembler with gcc-6 and gcc-9

[skein] Fix compilation on gnu assembler with gcc-6 and gcc-9

For some reason I don't want to really understand, the following
happens with gnu as.

/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S: Assembler messages:
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:466: Error: found '(', expected: ')'
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:466: Error: junk at end of line, first unrecognized character is `('
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:795: Error: found '(', expected: ')'
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:795: Error: junk at end of line, first unrecognized character is `('
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:885: Error: non-constant expression in ".if" statement
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:885: Error: non-constant expression in ".if" statement
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:885: Error: non-constant expression in ".if" statement
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:885: Error: non-constant expression in ".if" statement
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:885: Error: non-constant expression in ".if" statement
/home/adrian/git/freebsd/src/sys/crypto/skein/amd64/skein_block_asm.S:885: Error: non-constant expression in ".if" statement

After an exhaustive search and experimentation at 11pm, I discovered that
putting them in parentheses fixes the compilation.

Ed pointed out that I could likely fix this in a bunch of other
locations but I'd rather leave these alone until other options
are enabled.

Tested:

* gcc-6, amd64

Reviewed by:	emaste