[libc++] Fix regex_traits::isctype on big endian platforms (#73200)

'isctype' fails in arm64-big-endian because the __regex_word involved
in mask operation is not changed based on the platform endianness, while
the character mask does change.

Author: W-50243

libcxx/include/__locale

@@ -389,7 +389,7 @@ public:
     static const mask punct  = _ISpunct;
     static const mask xdigit = _ISxdigit;
     static const mask blank  = _ISblank;
-#if defined(__mips__)
+#if defined(__mips__) || (BYTE_ORDER == BIG_ENDIAN)
     static const mask __regex_word = static_cast<mask>(_ISbit(15));
 #else
     static const mask __regex_word = 0x80;