x/build: add builder that tests usage of gccgo to bootstrap go gc

[martisch]

There have been issues now and in the past about bootstrapping Go gc from gccgo being broken #40855, #30771.

Documentation at https://golang.org/doc/install/source#bootstrapFromGccgo suggests that bootstraping from gccgo is supported.

If this is the case I suggest we add a builder for that. To not be blocked on ressources I would assume a build only linux/amd64 builder would suffice to catch bootstrap issues.

/cc @ianlancetaylor @dmitshur

[dmitshur]

Given we say it is supported, I agree we should aim to have a test or a builder to confirm that it hasn't regressed.

We have some make.bash-only builders (e.g., search for CompileOnly in x/build/dashboard), so this can probably be arranged in a similar way.

Which version of Gccgo should be tested, and how often should we aim to update it?

/cc @cagedmantis @toothrot @andybons

[ianlancetaylor]

gccgo is released as part of GCC, and a new major version of GCC is released once a year.