MSYS,CYGWIN: Hard-code host system names when built for these runtimes

When CMake is built against the MSYS runtime library, `uname()` returns
a name that depends on the `MSYSTEM` environment variable.  Previously
we truncated `MSYS_...` to just `MSYS`, but outside `MSYSTEM=MSYS`
environments, names like `MINGW64_NT-10.0-22000` were reported.

The latter causes CMake to report an unsupported-platform error, which
users report as an issue when the real problem is that they should be
using a `MSYSTEM=MSYS` environment or a CMake that is not built against
the MSYS runtime.

For our purposes, if CMake is built against the MSYS runtime, the host
platform is always `MSYS`.  Similarly for `CYGWIN`.