misc/cgo/test: skip test7978 when using gccgo

Gccgo can only get a backtrace for the currently running thread, which
means that it can only get a backtrace for goroutines currently running
Go code.  When a goroutine is running C code, gccgo has no way to stop
it and get the backtrace.  This test is all about getting a backtrace
of goroutines running C code, so it can't work for gccgo.

Change-Id: I2dff4403841fb544da7396562ab1193875fc14c3
Reviewed-on: https://go-review.googlesource.com/1904
Reviewed-by: Minux Ma <minux@golang.org>