cmd/go/internal/work: update minimum supported s390x version on go

cmd/go/internal/work: update minimum supported s390x version on go

This updates cgo support for s390x changing from z196 to z13, as
z13 is the minimum machine level running on go for s390x.

Change-Id: I1a102294b2108c35ddb1428bf287ce83debaeac8
Reviewed-on: https://go-review.googlesource.com/c/go/+/666995
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>