Add art-linux-bionic-x64 target-config

This config can be used to run linux_bionic tests. Updated
run_build_test_target.py to support allowing fully custom build
instructions (Linux_Bionic cannot be built with a simple make
command).

Test: ./art/test/testrunner/run_build_test_target.py -j72 art-linux-bionic-x64
Bug: 118778408
Change-Id: I27d12c508407e318d31fc50e541f7d990118e7b6