Enable RSA AVX2 code.

This was disabled because we couldn't test it. We now have SDE for
testing which, even if it's not running on a builder yet, confirms that
this passes tests for all current and past Intel chips.

Change-Id: Iad74cc9944ee85557bb45c981751f84f335fb6c8
Reviewed-on: https://boringssl-review.googlesource.com/14010
Commit-Queue: Adam Langley <alangley@gmail.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/bn/asm/rsaz-avx2.pl

@@ -84,8 +84,8 @@ die "can't locate x86_64-xlate.pl";
 # output, so this isn't useful anyway.
 #
 # TODO(davidben): Enable these after testing. $avx goes up to 2 and $addx to 1.
-$avx = 0;
-$addx = 0;
+$avx = 2;
+$addx = 1;
 
 open OUT,"| \"$^X\" \"$xlate\" $flavour \"$output\"";
 *STDOUT = *OUT;