Get MSVC builds working on x86_64; add MSVC build targets that link the dynamic MSVC runtime

[DouglasDwyer]

This PR integrates the changes from #8 alongside some additional fixes. With these fixes, I am able to build the windows-msvc target on my machine.

In addition, this PR adds a new compilation flag (-Dmsvcrt_dynamic) that allows for linking the dynamic MSVC runtime (msvcrt.lib) rather than the static one (libcmt.lib). This is useful if you want to include machdxcompiler.lib in a project that already has other libraries linking to msvcrt.lib. I have also added CI builds that run with this flag enabled.

(Note: for easy proof that this is working, you can check out the CI builds on my fork of the project. The msvc targets for x86_64 now build!)

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