Build a dummy frame at the top of tmpstk to help debuggers trace the stack
when the system is idle.
swtch.s: idle():
Initialize the frame pointer so that debuggers don't try to trace a bogus
stack.
Load the frame pointer, load the stack pointer and switch out the old
stack in the unique order that never leaves one of the pointers pointers
invalid so that debuggers can trace idle(). Disabling interrupts
provides sufficient validity for normal operation, but debuggers use
(trace) traps.
54d024041c