runtime: testprogcgo: don't call exported Go functions directly from Go

runtime: testprogcgo: don't call exported Go functions directly from Go

Instead route through a C function, to avoid declaration conflicts
between the declaration needed in the cgo comment and the declaration
generated by cgo in _cgo_export.h.

This is not something user code will ever do, so no need to make it
work in cgo.

Fixes #46502

Change-Id: I1bfffdc76ef8ea63e3829871298d0774157957a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/327309
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>