runtime: fix *bsd/amd64 build

6g does not implement dead code elimination for const switches like it
does for const if statements, so the undefined raiseproc() function
was resulting in a link-time failure.

Change-Id: Ie4fcb3716cb4fe6e618033071df9de545ab3e0af
Reviewed-on: https://go-review.googlesource.com/2830


Reviewed-by: Russ Cox <rsc@golang.org>