x/build: support searching across build logs

[mdempsky]

For issues like #11617, it would be handy to be able to search across all build logs server-side rather than needing to download them locally.

[bradfitz]

I will be reducing my effort on the builders shortly so will be unable to commit to anything regarding this.

[mdempsky]

I discovered that https://www.google.com/search?q=site:build.golang.org+%22invalid+relocation%22 sort of works, though the search results page could be improved. Maybe one fix would be to serve the logs as HTML with appropriate titles and metadata to make for more useful snippets?

[bradfitz]

(bug scrub)

I still want to do this server-side, but note that @aclements has a client-side tool to do this. Austin, have you published it yet?

[aclements]

Sort of: https://godoc.org/github.com/aclements/go-misc/greplogs