x/build: add a Windows ARM32 builder

[zx2c4]

Per #42604 (comment) and #42604 (comment), my builder will be decommissioned quite soon. This issue is to track spinning up of a builder on Google infrastructure, using the new ARM64 systems to run GOOS=windows GOARCH=arm GOARM=7. My builder was just running the 32-bit tests on the normal 64-bit OS. The replacement builder can do the same. There shouldn't be much work to do for this, as it's already been done for #42604.

CC @toothrot @dmitshur @cagedmantis

[bcmills]

Duplicate of #47019?

[bcmills]

(I realize that this issue predates that one, but that one seems to have more recent updates. 😩)

[dmitshur]

Indeed. (I missed the earlier notification.) Closing this one in favor of #47019.