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.