Don't include .extern and .hidden on OS X.

Change-Id: I582eaa2ff922bbf1baf298a5c6857543524a8d4e
Reviewed-on: https://boringssl-review.googlesource.com/3810
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

crypto/perlasm/x86_64-xlate.pl

@@ -497,7 +497,11 @@ my %globals;
 		$self->{value} = $dir . "\t" . $line;
 
 		if ($dir =~ /\.extern/) {
-		    $self->{value} .= "\n.hidden $line";
+		    if ($flavour eq "elf") {
+			$self->{value} .= "\n.hidden $line";
+		    } else {
+			$self->{value} = "";
+		    }
 		} elsif (!$elf && $dir =~ /\.type/) {
 		    $self->{value} = "";
 		    $self->{value} = ".def\t" . ($globals{$1} or $1) . ";\t" .