runtime: fix sigfwd for aix/ppc64
runtime: fix sigfwd for aix/ppc64

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/+/164015
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

nawawi/go,josharian/go,golang/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,golang/go,josharian/go,cloudflare/go,akutz/go,golang/go,golang/go,akutz/go,josharian/go,CAFxX/go,akutz/go,josharian/go,akutz/go,golang/go,akutz/go,josharian/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,josharian/go,cloudflare/go,akutz/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,CAFxX/go,CAFxX/go,akutz/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,cloudflare/go,josharian/go,nawawi/go,josharian/go,nawawi/go