TST: Add Cygwin to the x86 feature tests.
TST: Add Cygwin to the x86 feature tests.

This was suggested by @seiko2plus for debugging a segfault in the
tests on Cygwin:
https://github.com/numpy/numpy/pull/18102#issuecomment-766429209

This test passes on Cygwin, and the whole testsuite has only the
failures I expect from running on Cygwin (see #18102 and #16246).

numpy/numpy,jakirkham/numpy,mhvk/numpy,charris/numpy,simongibbons/numpy,numpy/numpy,seberg/numpy,pdebuyl/numpy,rgommers/numpy,simongibbons/numpy,pdebuyl/numpy,endolith/numpy,anntzer/numpy,pdebuyl/numpy,mattip/numpy,mhvk/numpy,mattip/numpy,rgommers/numpy,jakirkham/numpy,simongibbons/numpy,mhvk/numpy,anntzer/numpy,seberg/numpy,charris/numpy,pdebuyl/numpy,rgommers/numpy,numpy/numpy,anntzer/numpy,numpy/numpy,rgommers/numpy,jakirkham/numpy,mhvk/numpy,simongibbons/numpy,endolith/numpy,jakirkham/numpy,mattip/numpy,jakirkham/numpy,seberg/numpy,charris/numpy,mhvk/numpy,endolith/numpy,endolith/numpy,simongibbons/numpy,mattip/numpy,charris/numpy,anntzer/numpy,seberg/numpy