Support a relative OUT_DIR

OUT_DIR may be a relative path to the top of the build tree.

Bug: 35984597
Test: OUT_DIR=out art/test/testrunner/testrunner.py --host -t 115-native-bridge
Test: OUT_DIR=$PWD/out art/test/testrunner/testrunner.py --host -t 115-native-bridge
Change-Id: I654ce26dbd45f0e2d54a0a0d5438c231372fe8f9
1 file changed