malloc: add M_PURGE mallopt flag

Add a new mallopt flag that purges any pending decaying pages.

Test: boots and works
bug 117795621

Exempt-From-Owner-Approval: approved on another branch
Merged-in: Ib250ae2b705b6a368c1efb801d6a7be54e075acb
Change-Id: Ib250ae2b705b6a368c1efb801d6a7be54e075acb