cmd/internal/obj/s390x: fix m6 field encoding for VSTRC instruction on s390x
M6 field for all extended mnemonics of VSTRC set to zero
This fixes VSTRC codegen to emit correctly and added testcases for all
the extended mnemonics.
Fixes #69216
Change-Id: I2a1b7fb61d6bd6444286eab56a506225c90b75e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/612315
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Vishwanatha HD <vishwanatha.hd@ibm.com> Reviewed-by: Carlos Amedee <carlos@golang.org>