Include .extern and .hidden in x86-64 asm.

It's unclear why .extern was being suppressed, it's also a little
unclear how the Chromium build was working without this. None the less,
it's causing problems with Android and it's more obviously correct to
make these symbols as hidden.

Change-Id: Id13ec238b80b8bd08d8ae923ac659835450e77f8
Reviewed-on: https://boringssl-review.googlesource.com/3800
Reviewed-by: Adam Langley <agl@google.com>

crypto/perlasm/x86_64-xlate.pl

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