x/build: add a Windows ARM builder

[cagedmantis]

We used to have a Windows ARM builder which was removed because it was no longer supported #38493. We should explore a new service or maintainer in order to restore the builder type.

/cc @andybons @dmitshur @toothrot

[gopherbot]

Change https://golang.org/cl/229559 mentions this issue: dashboard: remove the Windows ARM builder

[toothrot]

Referencing #36439 as the main issue for the Windows ARM port.

[zx2c4]

Most of you should have received a private correspondance from me with disk images and instructions so that it's easy to launch a Windows 10 ARM64 builder on EC2/GCP from any x86 machine using qemu. If you didn't receive that from me, and you're on the Google team configuring the new builder, please pipe up.

[cagedmantis]

@zx2c4 Thank you for the disk images. We will update the corresponding issues as soon as we begin working on them.