Mprotect from-space after uffd GC compaction

This is to avoid any possible prefetch accesses to the map, which could
cause issues when using memfd as the memfd is used on to-space in the
next GC cycle, and that avoids userfaults on the prefetched pages.

Bug: 160737021
Test: art/test/testrunner/testrunner.py --host --debug --gcstress
Change-Id: Ide61ae3842c52862b2257f2292a3d4ebd37db2f1