Configurations: PowerPC is big endian

Configurations: PowerPC is big endian

Define B_ENDIAN on PowerPC because it is a big endian architecture. With
this change the BN* related tests pass.

Fixes: #12199
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/12371)

(cherry picked from commit 52c6c12c1cad6f1046b34f4139d1aa3e967a5530)