This is in preparation of turning on escape analysis for the
    runtime.
    
    - In gccgo, systemstack is implemented with mcall, which is not
      go:noescape. Wrap the closure in noescape so the escape analysis
      does not think it escapes.
    
    - Mark some C functions go:noescape. They do not leak arguments.
    
    - Use noescape function to make a few local variables' addresses
      not escape. The escape analysis cannot figure out because they
      are assigned to pointer indirections.
    
    Reviewed-on: https://go-review.googlesource.com/86244

From-SVN: r256418