So if I enable this manually, I get various test failures from modules on OS X El Capitan with the same type of failure:

[ 95/398] test_bytes
Traceback (most recent call last):
  File "/Users/brettcannon/Repositories/python/cpython/3.5/Lib/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/Users/brettcannon/Repositories/python/cpython/3.5/Lib/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/Users/brettcannon/Repositories/python/cpython/3.5/Lib/test/__main__.py", line 3, in <module>
    regrtest.main_in_temp_cwd()
  File "/Users/brettcannon/Repositories/python/cpython/3.5/Lib/test/regrtest.py", line 1593, in main_in_temp_cwd
    main()
  File "/Users/brettcannon/Repositories/python/cpython/3.5/Lib/test/regrtest.py", line 756, in main
    raise Exception("Child error on {}: {}".format(test, result[1]))
Exception: Child error on test_bytes: Exit code -6

Since I can't get the test suite to pass with PGO+LTO I'm not comfortable committing this patch and promoting people turn them on.