Add fallback LE/BE for architectures with known endianness

Author: sipa

src/util.h

@@ -186,6 +186,14 @@ static SECP256K1_INLINE void *manual_alloc(void** prealloc_ptr, size_t alloc_siz
 #if defined(_MSC_VER) && defined(_WIN32) && !defined(SECP256K1_LITTLE_ENDIAN)
 # define SECP256K1_LITTLE_ENDIAN
 #endif
+#if !defined(SECP256K1_LITTLE_ENDIAN) && !defined(SECP256K1_BIG_ENDIAN)
+# if defined(__i386__) || defined(__i686__) || defined(__x86_64__) || defined(__amd64__) || defined(__ARMEL__) || defined(__AARCH64EL__) || defined(__MIPSEL__)
+#  define SECP256K1_LITTLE_ENDIAN
+# endif
+# if defined(__MIPSEB__) || defined(__ARMEB__) || defined(__AARCH64EB__)
+#  define SECP256K1_BIG_ENDIAN
+# endif
+#endif
 #if defined(SECP256K1_LITTLE_ENDIAN) == defined(SECP256K1_BIG_ENDIAN)
 # error Please make sure that either SECP256K1_LITTLE_ENDIAN or SECP256K1_BIG_ENDIAN is set, see src/util.h.
 #endif