x/build: add builder to test ios/amd64 port

[dmitshur]

If it is possible to create a builder for the ios/amd64 target (iOS Simulator, see #42100) with a reasonable amount of effort, we should create it. This is the tracking issue for any investigation and work that goes into this.

CC @golang/release, @cherrymui, @eliasnaur.

[gopherbot]

Change https://golang.org/cl/264738 mentions this issue: dashboard: skip ios-amd64 SlowBot alias