Distinguish between 32bit and 64bit cygwin platforms.

Add support for `cygwin32` and `cygwin64` platform values, and retire
having the one `cygwin` value. Update runtime, makefiles, test .skipifs,
and docs for the new values.

Cygwin has supported both 32- and 64-bit for some time. They can be
installed side-by-side on a single system. This update allows Chapel to
support both cygwin architectures.

I verified this works on two cygwin boxes, one with cygwin-x86 (aka
platform=cygwin32) and the other with cygwin-x86_64 (aka
platform==cygwin64). I ran nightly against examples just to make sure
nothing was fundamentally broken. I also ran
`start_test --futures test/io/ferguson/utf8/` on cygwin32, cygwin64, and
darwin and verified that .skipifs in that dir worked as expected.

PR #947 updates the test scripts to run the full suite of tests on both
platforms. That should highlight any additional issues (running the entire
suite takes all day, so we'll rely on the nightly testing).