Support a chroot-based environment in ART Buildbot's target harness.

When ART_TEST_CHROOT is defined, instead of having the ART Buildbot
install ART into a local path (set with ART_TEST_ANDROID_ROOT) and
hijack the linker (using CUSTOM_TARGET_LINKER), prepare and use a
chroot environment (set with ART_TEST_CHROOT).

In this scenario, ART and its dependencies can be built and used
normally (in particular, when building from the master-art Android
manifest) and still be executed as a standalone runtime outside of
the device's /system path, for testing purposes.

Test: Have the ART Buildbot build and run ART on device with chroot.
Bug: 34729697
Bug: 68125496
Change-Id: I08f1acd0d2813584f703fedb84e69df954cbdbda