Building Go shared libraries requires that all functions that have declarations
without bodies have implementations and vice versa, so remove the
implementation of call16 and add a stub implementation of sigreturn.

Change-Id: I4d5a30c8637a5da7991054e151a536611d5bea46
Reviewed-on: https://go-review.googlesource.com/15966Reviewed-by: Ian Lance Taylor <iant@golang.org>