ymm registers are not suffixed with w.

This imports a fix to x86gas.pl from upstream's
a98c648e40ea5158c8ba29b5a70ccc239d426a20. It's needed to get poly1305-x86.pl
working.

Confirmed that this is a no-op for our current assembly files.

Change-Id: I28de1dbf421b29a06147d1aea3ff3659372a78b3
Reviewed-on: https://boringssl-review.googlesource.com/7210
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/crypto/perlasm/x86gas.pl b/crypto/perlasm/x86gas.pl
index 99d7c1b..55d09d8 100644
--- a/crypto/perlasm/x86gas.pl
+++ b/crypto/perlasm/x86gas.pl
@@ -17,7 +17,7 @@
 { my $reg=shift;
     if    ($reg =~ m/^%e/o)		{ "l"; }
     elsif ($reg =~ m/^%[a-d][hl]$/o)	{ "b"; }
-    elsif ($reg =~ m/^%[xm]/o)		{ undef; }
+    elsif ($reg =~ m/^%[yxm]/o)		{ undef; }
     else				{ "w"; }
 }