hwpmc: split out PMC_FN_PROCESS_EXEC

Move the functionality into a separate helper function. All other
actions in pmc_hook_handler() already have this.

While here make some small style improvements. Restructure one for loop.

Reviewed by: jkoshy
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40333