Support win arm64 in rust source

[Ilia-Kosenkov]

This is a draft of a PR to add native support for ARM64 on Windows.
It also drops support for RTOOLS40, non-ucrt R and 32 bit architecture (subject to debate).

[Ilia-Kosenkov]

I will try to add proper unit tests later. The only remaining question is should I retire 32bit and pre-ucrt support.

[JosiahParry]

Can you elaborate on how this PR is supposed to work? I'm quite confused.

The use_rtools() is only used in rust_source(). Do we need to add other places e.g. in the makevars to have actual arm64 support, or do we already have that? just not for rust_source() and the knitr engine?

[codecov]

Codecov Report

❌ Patch coverage is 95.12195% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.48%. Comparing base (841b343) to head (a1c372b).

Files with missing lines	Patch %	Lines
R/source.R	0.00%	3 Missing ⚠️
R/use_rtools.R	98.73%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
R/use_rtools.R	98.73% <98.73%> (ø)
R/source.R	81.49% <0.00%> (+6.54%)	⬆️

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