cmd/cgo: support niladic macros for gccgo

cmd/cgo: support niladic macros for gccgo

https://golang.org/cl/43970 changed writeOutputFunc to support niladic
function-like macros; apply the corresponding change to
writeGccgoOutputFunc.

Updates #10715
Updates #18720

Change-Id: I5decb1d37ec71507466ade2eeda4b89c8785eaef
Reviewed-on: https://go-review.googlesource.com/86475
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Hiroshi Ioka <hirochachacha@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>