runtime: add missing return for non-GNU/Linux version of tgkill
runtime: add missing return for non-GNU/Linux version of tgkill

Path from Rainer Orth.

Change-Id: I40fc68a34a77a62c18eb5aa41a951c3885f55d37
Reviewed-on: https://go-review.googlesource.com/c/153118
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/gofrontend,golang/gofrontend,golang/gofrontend,golang/gofrontend