scripts/build-binary: Added s390x support for binary release

[nirmannarang]

scripts/build-binary: Added TARGET_ARCHS for supporting s390x binary build for release
.travis.yml: Added s390x build in all-build target.

[codecov-io]

Codecov Report

Merging #11358 into master will decrease coverage by 0.6%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11358      +/-   ##
==========================================
- Coverage   64.83%   64.23%   -0.61%     
==========================================
  Files         403      403              
  Lines       37969    37969              
==========================================
- Hits        24618    24389     -229     
- Misses      11723    11955     +232     
+ Partials     1628     1625       -3

Impacted Files	Coverage Δ
auth/store.go	47.38% <0%> (-20.95%)	⬇️
client/client.go	70.58% <0%> (-13.4%)	⬇️
proxy/grpcproxy/register.go	69.44% <0%> (-11.12%)	⬇️
pkg/tlsutil/tlsutil.go	86.2% <0%> (-6.9%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
pkg/fileutil/purge.go	66% <0%> (-6%)	⬇️
pkg/logutil/zap_grpc.go	47.61% <0%> (-4.77%)	⬇️
proxy/grpcproxy/watcher.go	89.79% <0%> (-4.09%)	⬇️
clientv3/leasing/txn.go	88.09% <0%> (-3.18%)	⬇️
clientv3/concurrency/election.go	79.68% <0%> (-2.35%)	⬇️
... and 9 more

---

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[wenjiaswe]

cc @xiang90 @hexfusion

[nirmannarang]

@xiang90 @hexfusion When can this PR be merged?

[xiang90]

lgtm