Add Windows x86 build job.
This will make it easier to start working on Hololens embedding work without having to deal with a broken build first.
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes
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It’s a compiler plugin that uses unstable compiler APIs
that are not on a path to stabilization.
With this changes, there is one less thing that might break
when we update the compiler. For example:
https://github.com/sfackler/rust-phf/pull/101
