arm64, powerpc: fix calculation of 'used' in GET_STACK_USAGE

We do not consider the space reserved for the pcb to be part of the
total kstack size, so it should not be included in the calculation of
the used stack size.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b02908b051c897117ebbc3dcb55848383b17c435)