Make libdexfile_external.so accessible from binaries and libraries in /system.

There are dependencies on libdexfile_external from some central libraries
that are widely used (b/123186083).

One example is vendor/bin/hw/android.hardware.media.omx@1.0-service, which
requries the link from "system" to "runtime" in the [vendor] section.

The direct dependants are libunwindstack and simpleperf, so it's enough to
link from namespaces containing /system/{lib,lib64,bin}.

Test: Flash and boot
Test: Flash and boot with Runtime APEX enabled (http://r.android.com/q/topic:art-move-libs-to-runtime-apex)
Bug: 123186083
Bug: 113373927
Change-Id: I081aa7392c875202acdaf1185c2ff28e17ac7e76