scripts: Added s390x support for docker image release

[nirmannarang]

Dockerfile-release.s390x: Added dockerfile for s390x
etcdmain/etcd.go: Added GOARCH for s390x for supported archs
scripts/build-release.sh: Added TARGET_ARCH for s390x
scripts/release: Added TARGET_ARCH for s390x

[codecov-io]

Codecov Report

Merging #11548 into master will decrease coverage by 0.81%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master   #11548      +/-   ##
==========================================
- Coverage   64.72%    63.9%   -0.82%     
==========================================
  Files         403      403              
  Lines       38097    38097              
==========================================
- Hits        24657    24347     -310     
- Misses      11801    12116     +315     
+ Partials     1639     1634       -5

Impacted Files	Coverage Δ
etcdmain/etcd.go	0% <0%> (ø)	⬆️
client/client.go	58.49% <0%> (-25.5%)	⬇️
auth/store.go	47.95% <0%> (-20.67%)	⬇️
etcdserver/api/v3rpc/util.go	51.61% <0%> (-16.13%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
etcdserver/api/v3rpc/lease.go	69.31% <0%> (-5.69%)	⬇️
clientv3/concurrency/mutex.go	62.16% <0%> (-5.41%)	⬇️
clientv3/leasing/cache.go	87.22% <0%> (-4.45%)	⬇️
pkg/testutil/recorder.go	77.77% <0%> (-3.71%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
... and 18 more

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[gyuho]

@nirmannarang Thanks for the contribution. Could you also create another PR to highlight this in CHANGELOG? https://github.com/etcd-io/etcd/blob/master/CHANGELOG-3.5.md