MdePkg RVCT: add definition of UNREACHABLE

The RVCT compiler in --gnu mode appears to simply strip of the __builtin
prefix when it encounters calls to __builtin_xxx() functions, and so
the __builtin_unreachable() we emit for GCC results in linker errors
regarding undefined references against 'unreachable()'.

So define UNREACHABLE() to a NOP instead.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Include/Arm/ProcessorBind.h

@@ -28,6 +28,13 @@
 #pragma pack()
 #endif
 
+//
+// RVCT does not support the __builtin_unreachable() macro
+//
+#ifdef __ARMCC_VERSION
+#define UNREACHABLE()
+#endif
+
 #if _MSC_EXTENSIONS 
   //
   // use Microsoft* C complier dependent integer width types