cmd/compile: document register-based ABI for s390x

cmd/compile: document register-based ABI for s390x

This CL adds the s390x information to the ABI doc.

Update #40724

Cq-Include-Trybots: luci.golang.try:gotip-linux-s390x
Change-Id: I1b4b25ef1003e2ab011e1b808aeb1c02288095c2
Reviewed-on: https://go-review.googlesource.com/c/go/+/719460
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Vishwanatha HD <vishwanatha.hd@ibm.com>
TryBot-Bypass: David Chase <drchase@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>