Load namespace configuration from ld.config.txt

This change allows customization of default namespace
configuration for different executables. It also enables
target_sdk_version setup for binaries (note that this
option should explicitly be enabled in ld.config.txt).

Bug: http://b/30706810
Bug: http://b/30435785
Test: run linker-unit-tests/bionic-unit-tests, boot angler
Change-Id: Ibbe87209acf1538fc9cec04944f3d22a190c38f1