x64: Allow building with LUAJIT_USE_SYSMALLOC and LUAJIT_USE_VALGRIND.

Valgrind 3.9 killed MAP_32BIT support. Ugh. So now we have to rely on
undocumented behavior where Valgrind always allocates from the bottom
of memory. Alas, such a binary won't run properly without Valgrind.

src/Makefile

@@ -122,8 +122,10 @@ XCFLAGS=
 #
 # Use the system provided memory allocator (realloc) instead of the
 # bundled memory allocator. This is slower, but sometimes helpful for
-# debugging. It's helpful for Valgrind's memcheck tool, too. This option
-# cannot be enabled on x64, since the built-in allocator is mandatory.
+# debugging. This option cannot be enabled on x64, since realloc usually
+# doesn't return addresses in the right address range.
+# OTOH this option is mandatory for Valgrind's memcheck tool on x64 and
+# the only way to get useful results from it for all other architectures.
 #XCFLAGS+= -DLUAJIT_USE_SYSMALLOC
 #
 # This define is required to run LuaJIT under Valgrind. The Valgrind