[SystemZ/zos] Define Endian constants for z/OS.

[Kai]

Summary

This is needed to build LLVM on z/OS, as there is no header file which provides these constants.

[hubert.reinterpretcast]

LGTM. This introduces just the second copy of the BIG_ENDIAN/LITTLE_ENDIAN defines, so I think it is fine not to try using a common copy at this time.

llvm/include/llvm/Support/SwapByteOrder.h

 #include <sys/types.h>
 #define BIG_ENDIAN 4321
 #define LITTLE_ENDIAN 1234
 #if defined(_BIG_ENDIAN)
 #define BYTE_ORDER BIG_ENDIAN
 #else
 #define BYTE_ORDER LITTLE_ENDIAN
 #endif
+#elif defined(__MVS__)
+#define BIG_ENDIAN 4321
+#define LITTLE_ENDIAN 1234
+#define BYTE_ORDER BIG_ENDIAN
 #else
 #if !defined(BYTE_ORDER) && !defined(_WIN32)
 #include <machine/endian.h>
 #endif
 #endif
 
 namespace llvm {