hwpmc: use kstack_contains()

This existing helper function is preferable to the hand-rolled
calculation of the kstack bounds.

Make some small style improvements while here. Notably, rename every
instance of "r", the return address, to "ra". Tidy the includes in the
affected files.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D39909

(cherry picked from commit aba91805aa92a47b2f3f01741a55ff9f07c42d04)