Add ld.config.txt to /system/etc

This file describes how loader should set up
default namespace for different kind of binaries.

Note that vendor and some of system binaries are
not yet ready for this config to be enabled - they
rely on libraries they shouldn't be relying upon.

Bug: http://b/30435785
Test: m
Change-Id: I7d5853a6b55db169be1dc2c38cc682711bf7f7f5