chacha/asm/chacha-ppc.pl: fix big-endian build.

It's kind of a "brown-bag" bug, as I did recognize the problem and
verified an ad-hoc solution, but failed to follow up with cross-checks
prior filing previous merge request.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6435)
