goprintf is a printf-like print for Go.
It is used in the code generated by 'defer print(...)' and 'go print(...)'.

Normally print(1, 2, 3) turns into

        printint(1)
        printint(2)
        printint(3)

but defer and go need a single function call to give the runtime;
they give the runtime something like goprintf("%d%d%d", 1, 2, 3).

Variadic functions like goprintf cannot be described in the new
type information world, so we have to replace it.

Replace with a custom function, so that defer print(1, 2, 3) turns
into

        defer func(a1, a2, a3 int) {
                print(a1, a2, a3)
        }(1, 2, 3)

(and then the print becomes three different printints as usual).

Fixes #8614.

LGTM=austin
R=austin
CC=golang-codereviews, r
https://golang.org/cl/159700043