runtime: don't check for stale runtime
runtime: don't check for stale runtime

The gccgo runtime is never stale, and on a system with gc sources in
~/go the test may wind up checking whether the gc runtime is stale.

Change-Id: I51935fa832c514daf3393e768a1f0eef855e84d5
Reviewed-on: https://go-review.googlesource.com/102282
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend