Kernel: Move userspace virtual address range base to 0x10000

Now that the shared bottom 2 MiB virtual address mappings are gone
userspace can use lower virtual addresses.

Kernel/API/MemoryLayout.h

@@ -0,0 +1,12 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+#include <Kernel/Sections.h>
+
+constexpr FlatPtr userspace_range_base = USER_RANGE_BASE;