runtime: don't build test files that use SetCgoTraceback for gccgo
The gofrontend doesn't support the runtime.SetCgoTraceback function,
which is specifically for handling mixed Go and C tracebacks.
Use a build tag to avoid compiling the runtime/testdata/testprogcgo
files that refer to SetCgoTraceback. These files are not currently
compiled anyhow, but they will be with a future gotools patch.
Reviewed-on: https://go-review.googlesource.com/46452
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@249556 138bc75d-0d04-0410-961f-82ee72b054a4