Add alignof wrapper for old compilers.

Older versions of MSVC don't support C++11's alignof(), but they do
support __alignof() which functions identically.

Likewise, XL C/C++ versions that predate xlclang/xlclang++ only
support __alignof__() (and optionally __alignof()).

Signed-off-by: Younes Manton <ymanton@ca.ibm.com>

include_core/omrcomp.h

@@ -601,4 +601,12 @@ typedef struct U_128 {
 #define OMR_LOG_POINTER_SIZE 2
 #endif /* defined(OMR_ENV_DATA64) */
 
+#if defined(_MSC_VER) && (1900 > _MSC_VER) /* MSVC versions prior to Visual Studio 2015 (14.0) */
+#define OMR_ALIGNOF(x) __alignof(x)
+#elif defined(__IBMC__) || defined(__IBMCPP__) /* XL C/C++ versions prior to xlclang/xlclang++ */
+#define OMR_ALIGNOF(x) __alignof__(x)
+#else /* All other compilers that support C11 and C++11 */
+#define OMR_ALIGNOF(x) alignof(x)
+#endif /* defined(_MSC_VER) && (1900 > _MSC_VER) */
+
 #endif /* OMRCOMP_H */