Support new tier 3 *-win7-windows-msvc targets

[varphone]

Two new targets have been added with Windows 7 as their baseline: x86_64-win7-windows-msvc and i686-win7-windows-msvc. They are starting as Tier 3 targets, meaning that the Rust codebase has support for them but we don't build or test them automatically. Once these targets reach Tier 2 status, they will be available to use via rustup.

https://blog.rust-lang.org/2024/02/26/Windows-7.html

[honsunrise]

Is this repository still being maintained?

[pkgw]

It's pretty much unmaintained at this point. I aspire to at least be responsive to pull requests, but even that is challenging these days, as you can presumably tell.

It's also true that vcpkg gets a lot of usage, so we're generally reluctant to make substantial changes, but this PR is quite straightforward. @waych if you don't object in a day or two, I'll go ahead and merge.