In 1.11 writebarrierptr is going away, so change the compiler to call
    gcWriteBarrier instead.  We weren't using gcWriteBarrier before;
    adjust the implementation to use the putFast method.
    
    This revealed a problem in the kickoff function.  When using cgo,
    kickoff can be called on the g0 of an m allocated by newExtraM.  In
    that case the m will generally have a p, but systemstack may be called
    by wbBufFlush as part of flushing the write barrier buffer.  At that
    point the buffer is full, so we can not do a write barrier.  So adjust
    the existing code in kickoff so that in the case where we are g0,
    don't do any write barrier at all.
    
    Reviewed-on: https://go-review.googlesource.com/131395

From-SVN: r264295