Manage KernelStacks separately rather than just allocing bytes.

Create a global KernelStackManager that will handle the relevant allocs.

zion/memory/virtual_memory.cpp

@@ -0,0 +1,18 @@
+#include "memory/virtual_memory.h"
+
+#include "memory/kernel_stack_manager.h"
+
+extern KernelStackManager* gKernelStackManager;
+
+uint64_t* VirtualMemory::AllocateKernelStack() {
+  return gKernelStackManager->AllocateKernelStack();
+}
+
+uint64_t VirtualMemory::GetNextMemMapAddr(uint64_t size) {
+  uint64_t addr = next_memmap_addr_;
+  next_memmap_addr_ += size;
+  if (next_memmap_addr_ >= 0x30'00000000) {
+    panic("OOM: Memmap");
+  }
+  return addr;
+}