chacha/asm/chacha-armv8.pl: fix intermittent build failures.

(Imported from b9077d85b0042d3d5d877d5cf7f06a8a8c035673.)

Change-Id: I6df3b3d0913b001712a78671c69b9468e059047f
Reviewed-on: https://boringssl-review.googlesource.com/7682
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>

crypto/chacha/asm/chacha-armv8.pl

@@ -1124,3 +1124,4 @@ foreach (split("\n",$code)) {
 
 	print $_,"\n";
 }
+close STDOUT;	# flush