In Go 1.17 the gc toolchain changed to set runtime.GOROOT in cmd/link
(previously it was runtime/internal/sys.GOROOT). Do the same in libgo.
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/351313
gotools/:
* Makefile.am (check-runtime): Add goroot.go to --extrafiles.
* Makefile.in: Regenerate.