Give a knob to have apex ignore system-lib special cases

apex will normally place libc and some other system libraries into a
separate directory. This is to ensure that early startup works
correctly. Some apex targets want to have these libraries at the
normal places however.

Test: ./art/tools/build_linux_bionic.sh com.android.runtime.host
Bug: 123591866
Bug: 120266448
Change-Id: Ib5a67a43fe3eea6188b4df9215e743f1634045f3