Fix AES-GCM-SIV endian issues

Fixes #18911

`BSWAP`x/`GETU`xx are no-ops on big-endian. Change the byte swapper.
Fix big-endian issues in the `mulx_ghash()` function

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18920)