Broaden abi selection for non-specified abi types.

[kwlzn]

Fixes #492

[kwlzn]

noting local success on the OSX specific integration test:

[omerta pex (kwlzn/492)]$ tox -e py27-requests -- -k test_cross_platform_abi_targeting_behavior
GLOB sdist-make: /Users/kwilson/dev/pex/setup.py
py27-requests inst-nodeps: /Users/kwilson/dev/pex/.tox/dist/pex-1.4.2.zip
py27-requests installed: certifi==2018.4.16,chardet==3.0.4,cookies==2.2.1,funcsigs==1.0.2,idna==2.6,mock==2.0.0,packaging==16.8,pbr==4.0.3,pex==1.4.2,py==1.5.3,pyparsing==2.2.0,pytest==2.9.1,requests==2.18.4,responses==0.9.0,six==1.11.0,twitter.common.contextutil==0.3.9,twitter.common.dirutil==0.3.9,twitter.common.lang==0.3.9,twitter.common.testing==0.3.9,urllib3==1.22
py27-requests runtests: PYTHONHASHSEED='1821441303'
py27-requests runtests: commands[0] | py.test -k not tests/test_integration.py -k test_cross_platform_abi_targeting_behavior
========================================================= test session starts ==========================================================
platform darwin -- Python 2.7.13, pytest-2.9.1, py-1.5.3, pluggy-0.3.1
rootdir: /Users/kwilson/dev/pex, inifile: 
collected 231 items 

tests/test_integration.py ..

================================ 229 tests deselected by '-ktest_cross_platform_abi_targeting_behavior' ================================
=============================================== 2 passed, 229 deselected in 0.95 seconds ===============================================
py27-requests runtests: commands[1] | pex --cache-dir /Users/kwilson/dev/pex/.tox/py27-requests/tmp/buildcache wheel requests . -e pex.bin.pex:main --version
pex 1.4.2
_______________________________________________________________ summary ________________________________________________________________
  py27-requests: commands succeeded
  congratulations :)

and TODO'd this with #479

[jsirois]

Duplicates line above.

[kwlzn]

thanks - fixed!