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`.

Source/cmSystemTools.cxx

@@ -3697,6 +3697,10 @@ cm::string_view cmSystemTools::GetSystemName()
 {
 #if defined(_WIN32)
   return "Windows";
+#elif defined(__MSYS__)
+  return "MSYS";
+#elif defined(__CYGWIN__)
+  return "CYGWIN";
 #elif defined(__ANDROID__)
   return "Android";
 #else
@@ -3725,15 +3729,6 @@ cm::string_view cmSystemTools::GetSystemName()
     if (systemName.find("kFreeBSD") != cm::string_view::npos) {
       systemName = "kFreeBSD";
     }
-
-    // fix for CYGWIN and MSYS which have windows version in them
-    if (systemName.find("CYGWIN") != cm::string_view::npos) {
-      systemName = "CYGWIN";
-    }
-
-    if (systemName.find("MSYS") != cm::string_view::npos) {
-      systemName = "MSYS";
-    }
     return systemName;
   }
   return "";