x/build: add a builder that runs long tests with a gccgo toolchain

[jayconrod]

From the list at build.golang.org, there don't seem to be any builders running a gccgo toolchain. It would be useful to have one to make sure we don't regress on gccgo support.

For quick reference: gccgo setup instructions

[bcmills]

This missing build configuration masked a failure in a new test (#42688).

(FYI @golang/release for prioritization.)

[bcmills]

Another masked failure, apparently since Dec. 9: #43974.

[gopherbot]

Change https://golang.org/cl/287613 mentions this issue: cmd/go: revert TestScript/build_trimpath to use ioutil.ReadFile

[bcmills]

This masked another failure in CL 416134 — longtest SlowBots were run for that CL, but the only builders that detected the broken test were for secondary ports (aix, illumos, and solaris).

[gopherbot]

Change https://go.dev/cl/417094 mentions this issue: cmd/go: avoid indexing GOROOT packages when the compiler is 'gccgo'

[heschi]

@bcmills we're probably not going to get to this any time soon. But we're happy to work with you or anyone who feels that it's important to get it set up.