build: Endian compatibility

- Detect endian instead of stopping configure on big-endian
- Add `byteswap.h` and `endian.h` header for compatibility with
  Windows and other operating systems that don't come with them
- Update `crypto/common.h` functions to use compat
  endian header

src/Makefile.am

@@ -141,6 +141,8 @@ BITCOIN_CORE_H = \
   walletdb.h \
   wallet.h \
   wallet_ismine.h \
+  compat/byteswap.h \
+  compat/endian.h \
   compat/sanity.h
 
 JSON_H = \