Make native GC-root updation concurrent with userfaultfd

Additionally also uses userfaultfd's minor-fault feature for moving
space.

Bug: 160737021
Test: ART_USE_READ_BARRIER=false art/test/testrunner/testrunner.py and module install
Change-Id: I98b0c69fba4aec1263b1f38cc9f31494fd5c8cf5
15 files changed