MIPS assembly pack: default heuristic detection to little-endian.

Current endianness detection is somewhat opportunistic and can fail
in cross-compile scenario. Since we are more likely to cross-compile
for little-endian now, adjust the default accordingly.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5613)