This definition enables callers to estimate remaining space on the
kstack, and take action on it. Notably, it enables optimizations in the
GEOM and netgraph subsystems to directly dispatch work items when there
is sufficient stack space, rather than queuing them for a worker thread.

Implement it for riscv, arm, and mips. Remove the #ifdefs, so it will
not go unimplemented elsewhere.

PR:		259157
Reviewed by:	mav, kib, markj (previous version)
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D32580