Manage KernelStacks separately rather than just allocing bytes.

Create a global KernelStackManager that will handle the relevant allocs.

zion/memory/kernel_stack_manager.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <stdint.h>
+
+// KernelStackManager doles out kernel stacks.
+//
+// KernelStacks are in the region:
+// 0xFFFFFFFF 90000000 - 0xFFFFFFFF 9FFFFFFF
+//
+// Each kernel stack is 12 KiB with a 4 Kib page boundary.
+//
+// It is  global object that is only exposed via internal linkage
+// to the VirtualMemory class. All kernel stacks should be created through that
+// class.
+class KernelStackManager {
+ public:
+  static void Init();
+
+  uint64_t* AllocateKernelStack();
+
+  void FreeKernelStack(uint64_t stack_base);
+
+ private:
+  KernelStackManager();
+  uint64_t next_stack_addr_;
+  uint64_t freed_stack_cnt_ = 0;
+};