MemoryUtil: add comment for Valgrind

Valgrind doesn't support the mmap() flag MAP_32BIT. Adding a simple
CMake option would force a recompile, so just add this comment.

Source/Core/Common/MemoryUtil.cpp

@@ -21,6 +21,10 @@
 #include <sys/mman.h>
 #endif
 
+// Valgrind doesn't support MAP_32BIT.
+// Uncomment the following line to be able to run Dolphin in Valgrind.
+//#undef MAP_32BIT
+
 #if !defined(_WIN32) && defined(_M_X86_64) && !defined(MAP_32BIT)
 #include <unistd.h>
 #define PAGE_MASK     (getpagesize() - 1)