Also always access the atomicstatus field atomically.
    
    The effect of not checking the _Gscan bit is that if the GC decides to
    scan the stack just as the goroutine is leaving the system call, the
    goroutine might fail to call exitsyscall.  Then then typically causes
    a runtime assertion failure later on.  If we do call exitsyscall as we
    should, it will stall (in casgstatus) until the _Gscan bit is cleared.
    
    No separate test.  I've observed causing sporadic failures running the
    misc/cgo tests, but we don't currently have a way to run those
    routinely for gccgo.  I should fix that.
    
    Reviewed-on: https://go-review.googlesource.com/45392

From-SVN: r249138