SystemInformation: Improve CPU info lookup on Cygwin

Based on downstream patch from Cygwin package for CMake by Marco Atzeri:

* https://github.com/matzeri/cygwin-pkg/blob/64864eb8f0d635ea44226e4b9a41a7ca59a1c7dd/cmake/3.17.3-cpuinfo.patch

and a patch by Lemures Lemniscati

* https://sourceware.org/pipermail/cygwin/2020-July/245563.html