Gate assembly sources on !OPENSSL_NO_ASM.

Change-Id: I32b37306265e89afca568f20bfba2e04559c4f0b
Reviewed-on: https://boringssl-review.googlesource.com/14527
Commit-Queue: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/perlasm/x86_64-xlate.pl

@@ -1134,7 +1134,7 @@ ___
 OPTION	DOTNAME
 ___
 }
-print STDOUT "#if defined(__x86_64__)\n" if ($gas);
+print STDOUT "#if defined(__x86_64__) && !defined(OPENSSL_NO_ASM)\n" if ($gas);
 
 while(defined(my $line=<>)) {