Currently, cgo supports only macros which can be reduced to constants
or variables. The CL addresses remaining parts, macros which can be
represented as niladic functions.

The basic idea is simple:
  1. make a thin wrapper function per macros.
  2. replace macro expansions with function calls.

Fixes #10715
Fixes #18720

Change-Id: I150b4fb48e9dc4cc34466ef6417c04ac93d4bc1a
Reviewed-on: https://go-review.googlesource.com/43970
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>