runtime: fix isSystemGoroutine for gccgo

runtime: fix isSystemGoroutine for gccgo

    The gc toolchain decides whether a goroutine is a system goroutine by
    comparing startpc to a list of saved special PCs.  In gccgo that
    approach does not work as startpc is often a thunk that invokes the
    real function with arguments, so the thunk address never matches the
    saved special PCs.

    This patch fixes gccgo's understanding of system goroutines.  Since
    there are only a limited number of them, we simply change each one to
    mark itself as special.

    This fixes stack dumps and functions like runtime.NumGoroutine to
    behave more like gc.  It also fixes the goprint test in the gc
    testsuite.

    Reviewed-on: https://go-review.googlesource.com/43156

From-SVN: r247931