On AIX, shared objects must be wrapped under an archive file.

For testso, creating libcgosotest with an extern symbol isn't
AIX-friendly. By default, ld will block such behavior. Rather than
forcing ld to work as on Linux and using the run-time linking,
goCallback became a function pointer which is set by setCallback().

Updates #30565

Change-Id: I455ab32faddd41f1b0c84cc9e503788044ad49b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/169020
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>