This commit fixes runtime.sigfwd for aix/ppc64. fn is a function
descriptor and not a function. R2 must be saved and restored.

Change-Id: Ie506b0bdde562ca37202d19973ba1d537c3d64e2
Reviewed-on: https://go-review.googlesource.com/c/go/+/164015Reviewed-by: Ian Lance Taylor <iant@golang.org>