Kernel: Define constant USERSPACE_END

This should be used for userspace generic allocations. Currently I used
KERNEL_OFFSET, but I want to limit userspace to the actual lower half of
the address space

kernel/include/kernel/Memory/Types.h

@@ -4,8 +4,10 @@
 
 #if ARCH(x86_64)
 	#define KERNEL_OFFSET 0xFFFFFFFF80000000
+	#define USERSPACE_END 0xFFFF800000000000
 #elif ARCH(i686)
 	#define KERNEL_OFFSET 0xC0000000
+	#define USERSPACE_END 0xC0000000
 #else
 	#error
 #endif