On other systems we use "SWI $n". Change Plan 9 files to be
consistent. Generated binary is unchanged.

Fixes #20378.

Change-Id: Ia2a722061da2450c7b30cb707ed4f172fafecf74
Reviewed-on: https://go-review.googlesource.com/43533Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>