x/all: gccgo support

[Helflym]

In #31382, @mikioh asks an interesting questions about the maintenance of golang.org/x packages on gccgo.

How are we sure that the changes made in golang.org/x will work on gccgo ?

For Go toolchain, they are builders on every GOOS/GOARCH which check that it's working but what about the Gccgo toolchain ? Do we assume that if it works on Go, it must work on Gccgo ?
The core code of each package should indeed work, after all that's just Go code. But what about special gccgo files, like in x/sys/unix for syscalls, or all the Go assembly files which cannot be built on gccgo, like in x/crypto ?

CC @bradfitz @ianlancetaylor

[bcmills]

CC @thanm

[ianlancetaylor]

At present we have no formal testing that the x/ packages work with gccgo. In general we have no formal testing of gccgo at all. Yes, it would be nice to fix this.

That said, most of the x/ packages do work with gccgo.