Turn off XOM on libnstest_root.so

The dlext.ns_anonymous test copies the loaded segments of this shared
object into a new mapping, so every segment must be readable. Turn off
eXecute-Only-Memory.

Bug: http://b/123034666
Test: bionic-unit-tests --gtest_filter=dlext.ns_anonymous
Change-Id: I2d427feb81d353e0403c7ef251a55afb4d8729b4