provide getstackpcs() that takes an ebp to start unwinding from

getcallerpcs() uses a void* to the first argument, but on x64 that
doesn't work (arguments passed in registers), so we just use the
actual rbp and unwind from there.  But for cases where we want to
unwind somebody else's stack (procdump()), we need a way to handle
that sanely.
another 64bit header update
make procdump() work on x64

Use the newly added getstackpcs() for arbitrary-bp-relative unwinding.
console: reboot on ctrl-z