[build] auto detect big endian (#4551)

configure.ac

@@ -202,6 +202,8 @@ AC_PROG_CXX_C_O
 
 AM_PROG_AS
 
+AC_C_BIGENDIAN
+
 # Check for other compiler toolchain tools.
 
 AC_CHECK_TOOL(AR, ar)

src/core/common/encoding.hpp

@@ -37,8 +37,13 @@
 #include "openthread-core-config.h"
 
 #ifndef BYTE_ORDER_BIG_ENDIAN
+#if defined(WORDS_BIGENDIAN) || \
+    defined(__BYTE_ORDER__) && defined(__ORDER_BIG_ENDIAN__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+#define BYTE_ORDER_BIG_ENDIAN 1
+#else
 #define BYTE_ORDER_BIG_ENDIAN 0
 #endif
+#endif
 
 #include <limits.h>
 #include <stdint.h>