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

[emaste]

Summary

Clang IAS does not support the --defsym argument, and
.ifndef SKEIN_USE_ASM
gets turned into
.ifndef 1792
by the preprocessor, which results in an error

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

GNU as works with both #ifdef and .ifdef.

[arichardson]

The other option would have been to change the build system to pass -Wa,-defsym, SKEIN_USE_ASM=XXX (that's what I did for my godbolt testing.
But either solution seems fine.

[emaste]

In D25154#554268, @arichardson wrote:

The other option would have been to change the build system to pass -Wa,-defsym, SKEIN_USE_ASM=XXX (that's what I did for my godbolt testing.
But either solution seems fine.

Oh hmm, I tried that and had an error, but now I see I tried --defsym with two dashes.

[arichardson]

In D25154#554275, @emaste wrote:

In D25154#554268, @arichardson wrote:

The other option would have been to change the build system to pass -Wa,-defsym, SKEIN_USE_ASM=XXX (that's what I did for my godbolt testing.
But either solution seems fine.

Oh hmm, I tried that and had an error, but now I see I tried --defsym with two dashes.

I haven't checked if there's an open LLVM bug, but clang should probably accept the -- version for compatibility.