cmd/internal/obj/s390x: make assembler almost concurrency-safe

CL 39922 made the arm assembler concurrency-safe.
This CL does the same, but for s390x.
The approach is similar: introduce ctxtz to hold
function-local state and thread it through
the assembler as necessary.

One race remains after this CL, similar to CL 40252.

That race is conceptually unrelated to this refactoring,
and will be addressed in a separate CL.

Passes toolstash-check -all.

Updates #15756

Change-Id: Iabf17aa242b70c0b078c2e85dae3d93a5e512372
Reviewed-on: https://go-review.googlesource.com/40371
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Munday <munday@ca.ibm.com>

src/cmd/internal/obj/link.go

@@ -727,7 +727,6 @@ type Link struct {
 	InlTree       InlTree // global inlining tree used by gc/inl.go
 	Imports       []string
 	Plan9privates *LSym
-	Printp        *Prog
 	Instoffset    int64
 	Autosize      int32
 	Pc            int64