perlasm/x86_64-xlate.pl: work around problem with hex constants in masm.

Perl, multiple versions, for some reason occasionally takes issue with
letter b[?] in ox([0-9a-f]+) regex. As result some constants, such as
0xb1 came out wrong when generating code for MASM. Fixes upstream
GH#3241.

(Imported from upstream's c47aea8af1e28e46e1ad5e2e7468b49fec3f4f29.)

This does not affect of the configurations we generate and is imported
to avoid a diff against upstream.

Change-Id: Iacde0ca5220c3607681fad081fbe72d8d613518f
Reviewed-on: https://boringssl-review.googlesource.com/15985
Reviewed-by: Adam Langley <agl@google.com>

crypto/perlasm/x86_64-xlate.pl

@@ -207,8 +207,9 @@ my %globals;
 	    }
 	    sprintf "\$%s",$self->{value};
 	} else {
-	    $self->{value} =~ s/0x([0-9a-f]+)/0$1h/ig if ($masm);
-	    sprintf "%s",$self->{value};
+	    my $value = $self->{value};
+	    $value =~ s/0x([0-9a-f]+)/0$1h/ig if ($masm);
+	    sprintf "%s",$value;
 	}
     }
 }