x/build: add a Windows ARM64 builder

[cagedmantis]

A builder should be added in support of the Windows/arm64 port #36439

Note: this is independent of #38607 which adds a Windows arm builder.

@golang/release

[zx2c4]

I have a ARM64 builder now, with a preinstalled gcc for each architecture supported on the machine:

[zx2c4]

CC @aclements @alexbrainman @jstarks

[gopherbot]

Change https://golang.org/cl/272106 mentions this issue: dashboard: add new Windows ARM builder