Expand default RTools toolchain usage to all R versions

[andrjohns]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

I had a look at the original Math PR adding compatibility with RTools make, and realised that it was compatible with all versions of RTools - not just RTools44!

This PR expands #1054 to use the stock Rtools toolchain for all cmdstan 2.35+ installations on Windows

Copyright and Licensing

Please list the copyright holder for the work you are submitting
(this will be you or your assignee, such as a university or company):
Andrew Johnson

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

Awesome, thanks! I just merged master into this branch after merging your other PR that fixed the failures on devel. If it all passes now I think we can go ahead and merge this.