Add limited target-specific configuration to apex.

Some apex targets need to be able to change their contents based on if
they are host or target or what libc they are using. This adds support
for doing this using the standard 'target: {...}' idiom.

Test: m com.android.runtime.host
Test: ./art/tools/build_linux_bionic.sh com.android.runtime.host
Bug: 123591866
Change-Id: If73bee650cdeb277c0e603763aa0b0108656bfdd