Mark executable maps with no name as unreadable.

On some devices, executable maps without names are created and deleted.
Doing unwinds can sometimes attempt to access these maps and crash because
the map has disappeared since that last read of the map.

Mark these maps as unreadable to avoid this problem.

Bug: 27152097
Change-Id: I17277823c1793a9d695d53cc6de3df53242c80a2