x/build: add automated notifications for missing builders to its owner and/or golang-dev

[minux]

Should the dashbaord send an email to the builder's owner
and/or golang-dev when a builder stops sending results for
more than some time?

The linux/arm64 builder is currently missing, from Feb 24,
so is netbsd/386 builder.

At the very least, the builder dashboard should show which
builders are missing on the web page.

[gopherbot]

CL https://golang.org/cl/20621 mentions this issue.

[dmitshur]

CC @golang/release.

[dmitshur]

In #50299 (comment), @changkun also suggested this:

Perhaps it would be better to have an email notification to the builder maintainer from farmer.golang.org when a builder is gone for 24 hours.

[dmitshur]

Watchflakes has started to automatically report missing builders (CC @cagedmantis).
Closing this in favor of the newer issue #68790.