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/lib_aux.c

@@ -302,7 +302,7 @@ static int panic(lua_State *L)
 
 #ifdef LUAJIT_USE_SYSMALLOC
 
-#if LJ_64
+#if LJ_64 && !defined(LUAJIT_USE_VALGRIND)
 #error "Must use builtin allocator for 64 bit target"
 #endif