Linker namespace configuration for the Runtime APEX.

Needs to support e.g. app_process in "default" -> libnativehelper in
"runtime" -> platform libs in "default" -> libc etc in "runtime".

Always make sure to switch namespaces when switching library paths between
/system and the APEX, so that internal library dependencies in both
locations are loaded from their own directory. E.g. libc++ and libbase live
in both places, and their dependents must always load the version they were
compiled with.

Bug: 119867084,113373927
Test: Flash and reboot device
Test: Remount / read-write, then on device: rm -f system/lib{,64}/lib{c,m,dl}.so; system/bin/dex2oat -h; stop && start
Test: mmma bionic/tests/ && adb root && adb shell rm -rf /data/nativetest\* && adb push out/target/product/taimen/data/nativetest* /data/ && adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests && adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests
Test: cts-tradefed run commandAndExit cts -m CtsJniTestCases
Change-Id: I6c4fc2f1abea1fb8851546e258ffaaa4ef6fe7d0