[SECP256K1] Add fallback LE/BE for architectures with known endianness + SHA256 selftest
Summary:
These are all the architecture macros I could find with known endianness. Use those as a fallback when BYTE_ORDER isn't available. See #787 (comment) It also adds a SHA256 selftest, so that improperly overriding the endianness detection will be detected at runtime.
Backport of secp256k1 PR799.
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8036