Common: Add proper macros to test for architecture pointer size

The old system of just defining macros available in some other platform
was susceptible to silently using the wrong code if you forgot to
include a particular header. This fixes a crash on non-Windows platforms
introduced by e1fbac3ca1.

src/common/memory_util.cpp

@@ -71,7 +71,7 @@ void* AllocateExecutableMemory(size_t size, bool low)
     }
 #endif
 
-#if defined(_M_X64)
+#if EMU_ARCH_BITS == 64
     if ((u64)ptr >= 0x80000000 && low == true)
         LOG_ERROR(Common_Memory, "Executable memory ended up above 2GB!");
 #endif