Linker namespace configuration for the Runtime APEX.

This mimics the namespace configuration in
system/core/rootdir/etc/ld.config.txt so that the same namespace separation
between APEX and /system libraries is kept. The "default" namespace here
corresponds to "runtime" in the system config, and "platform" here
corresponds to "default" there.

Bug: 119867084,113373927
Test: Flash and reboot device
Test: Run "/apex/com.android.runtime/bin/dex2oat -h" and check that libraries are loaded from right locations
Test: Run "adb shell cmd package compile -m speed-profile -f -a" with aosp/812733 to pick dex2oat in the APEX
Change-Id: Ibf3af74a1970076d1c03419fd6c64f35ca541e36