runtime: avoid write barriers when calling deferred function

runtime: avoid write barriers when calling deferred function

    Calling a deferred function currently requires changing from a uintptr
    to the function code to a Go function value. That is done by setting
    the value of a func local variable using unsafe.Pointer. The local
    variable will always be on the stack. Adjust the code that sets the
    local variable to avoid generating a write barrier.

    A write barrier is never needed here. Also, for deferreturn, we must
    avoid write barriers entirely when called from a cgo function; that
    requires more than just this, but this is a start.

    The test for this is runtime tests that use the go tool; these are not
    currently run, but they will be in the future.

    Reviewed-on: https://go-review.googlesource.com/46455

From-SVN: r249559