Fix handling of the "0:" label in arm-xlate.pl

When $label == "0", $label is not truthy, so `if ($label)` thinks there isn't
a label. Correct this by looking at the result of the s/// command.

Verified that there are no changes in the .S files created during a normal
build, and that the "0:" labels appear in the translation given in the error
report (and they are the only difference in the before and after output).

Fixes #21647

Change-Id: I5f2440100c62360bf4bdb7c7ece8dddd32553c79

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #21653)

(cherry picked from commit 9607f5c)

crypto/perlasm/arm-xlate.pl

@@ -159,9 +159,8 @@ sub expand_line {
     }
 
     {
-	$line =~ s|(^[\.\w]+)\:\s*||;
-	my $label = $1;
-	if ($label) {
+	if ($line =~ s|(^[\.\w]+)\:\s*||) {
+	    my $label = $1;
 	    printf "%s:",($GLOBALS{$label} or $label);
 	}
     }