Apply MADV_FREE to exec_map entries only after a lowmem event.

This effectively provides the same benefit as applying MADV_FREE inline
upon every execve, since the page daemon invokes lowmem handlers prior to
scanning the inactive queue. It also has less overhead; the cost of
applying MADV_FREE is very noticeable on many-CPU systems since it includes
that of a TLB shootdown of global PTEs. For instance, this change nearly
halves the system CPU usage during a buildkernel on a 128-vCPU EC2
instance (with some other patches applied).

Benchmarked by: cperciva (earlier version)
Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D9586