Allow selecting poll strategy, start to stub ev_poll_posix.c

src/core/iomgr/ev_poll_posix.c

src/core/iomgr/ev_poll_posix.h

src/core/iomgr/ev_posix.c

test/core/end2end/gen_build_yaml.py

tools/run_tests/run_tests.py

tools/run_tests/tests.json