i386: take pcb and fpu area into account in GET_STACK_USAGE

On this platform, the pcb and FPU save area are allocated from the top
of each kernel stack, so they should be excluded from the calculation of
the total and used stack sizes.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32581