Simplify task switches

No longer using the rsp from the entry to the kernel, but instead
switching rsp at task-switching time in assembly.

This currently breaks fork()

src/kernel/syscall.h

@@ -21,5 +21,5 @@ enum class syscall : uint64_t
 };
 
 void syscall_enable();
-uintptr_t syscall_dispatch(uintptr_t, cpu_state &);
+void syscall_dispatch(cpu_state *);