loader: enable loading libraries from tmpfs

This change adds two tests for dlopen from temporary files.
1. One Uses memfd_create() can be used to load libraries directly
from memory. This requires relaxing namespace accessibility check
in order to make this work in isolated namespaces.
2. Another checks that open with O_TMPFILE works.

Bug: http://b/37245203
Test: bionic-unit-tests --gtest_filter=dl*:Dl*
Change-Id: I3be1d7198ca17e7f1ba022a0d86c64d59a493506
(cherry picked from commit bb8b22a0)