Add write_cr3()

src/common/hos/cpu.d

@@ -15,6 +15,11 @@ ulong read_cr3()
     return __asm!ulong("mov %cr3, %rax", "={rax}");
 }
 
+void write_cr3(ulong v)
+{
+    __asm("mov $0, %cr3", "r", v);
+}
+
 ulong read_cr4()
 {
     return __asm!ulong("mov %cr4, %rax", "={rax}");