cmd/internal/obj/s390x: remove support for stores of global addresses

cmd/internal/obj/s390x: remove support for stores of global addresses

This CL removes support for MOVD instructions that store the address
of a global variable. For example:

  MOVD $main·a(SB), (R1)
  MOVD $main·b(SB), main·c(SB)

These instructions are emulated and the new backend doesn't need them
(the stores now always go through an intermediate register).

Change-Id: I3a1bcb3f19c5096ad0426afd76d35a4d7975733b
Reviewed-on: https://go-review.googlesource.com/30720
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>