cmd/internal/obj/s390x: remove param field from optab

The param field isn't useful, we can just use REGSP instead.

Change-Id: I2ac68131c390209cc84e43aa7620ccbf5ae69120
Reviewed-on: https://go-review.googlesource.com/c/go/+/171725
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>