Improve the region space memory mapping.

Add a region space mmap fallback when the initial address isn't
available.

Clean up around the asan-specific base address.

Add MemMap::AlignBy to align the region space base address by the
region size, which is currently required by ReadBarrierTable.

Disable some read barriers in ZygoteCompactingCollector to avoid a
DCHECK failure in LockWord::SetMarkBitState when classes are in the
forward state due to unnecessary read barriers on
SizeOf/VisitReference.

Bug: 12687968
Test: test-art-host with CC and CMS.
Test: marlin-userdebug_asan_coverage boot.
Test: angler boots with CC and CMS.

Change-Id: I70f99779df6acc1b64cab6402f3ef7c73ce5b39b