cmd/compile/internal/ssa/gen: set wasm genfile

This appears to have been an oversight and/or
left over from development.

Setting the genfile means that extra sanity
checks are executed when regenerating SSA files.
They already pass.

Change-Id: Icc01ecf85020d3d51355e8bccfbc521b52371747
Reviewed-on: https://go-review.googlesource.com/c/154459


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>