Fix x86_64-xlate.pl comment regex.

This did not correctly capture lines like the following:

https://boringssl.googlesource.com/boringssl/+/refs/heads/master/crypto/chacha/asm/chacha-x86_64.pl#260
https://boringssl.googlesource.com/boringssl/+/refs/heads/master/crypto/fipsmodule/aes/asm/aes-x86_64.pl#992
https://boringssl.googlesource.com/boringssl/+/refs/heads/master/crypto/fipsmodule/aes/asm/aesni-x86_64.pl#641
https://boringssl.googlesource.com/boringssl/+/refs/heads/master/crypto/fipsmodule/aes/asm/bsaes-x86_64.pl#387
https://boringssl.googlesource.com/boringssl/+/refs/heads/master/crypto/fipsmodule/modes/asm/ghash-x86_64.pl#455
https://boringssl.googlesource.com/boringssl/+/refs/heads/master/crypto/fipsmodule/ec/asm/p256-x86_64-asm.pl#92

Reportedly that last one causes problems with some assemblers.

Change-Id: I82d6f0d81b902e48fad3c45947f84f02370eb1ab
Reviewed-on: https://boringssl-review.googlesource.com/c/34925
Reviewed-by: Adam Langley <agl@google.com>

crypto/perlasm/x86_64-xlate.pl

@@ -1188,8 +1188,11 @@ while(defined(my $line=<>)) {
 	$line =~ s|[#!].*$||;	# get rid of asm-style comments...
     } else {
 	# Get rid of asm-style comments but not preprocessor directives. The
-	# latter are identified by not having a space after the '#'.
-	$line =~ s|[#!] .*$||;
+	# former are identified by having a letter after the '#' and starting in
+	# the first column.
+	$line =~ s|!.*$||;
+	$line =~ s|(?<=.)#.*$||;
+	$line =~ s|^#([^a-z].*)?$||;
     }
 
     $line =~ s|/\*.*\*/||;	# ... and C-style comments...