cmd/compile: cleanup genwrapper slightly

cmd/compile: cleanup genwrapper slightly

ORETJMP doesn't need an ONAME if we just set the target method on Sym
instead of Left. Conveniently, this is where fmt.go was looking for it
anyway.

Change the iface parameter and global compiling_wrappers to bool.

Passes toolstash-check.

Change-Id: I5333f8bcb4e06bf8161808041125eb95c439aafe
Reviewed-on: https://go-review.googlesource.com/68252
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>