Add and use internal header that implements endianness check

Add and use internal header that implements endianness check

This moves test/ossl_test_endian.h to include/internal/endian.h and
thereby makes the macros in there our standard way to check endianness
in run-time.

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/12390)