Summary:
Fix g++ -march=native detection and reenable s390x in travis
This PR fixes s390x assembler messages:
```
Error: invalid switch -march=z14
Error: unrecognized option -march=z14
```
The s390x travis build was failing with gcc-7 because the assembler on
ubuntu 16.04 is too old to recognize the z14 model so it doesn't work
with -march=native on a z14 machine. It fixes the check for the
-march=native flag so that the assembler will get called and correctly
fail on ubuntu 16.04 which will cause the build to fall back to
-march=z196 which works.
The other changes are needed so builds work more consistently on
s390x:
1. Set make parallelism to 1 for s390x: The default was 4 previously
but I saw frequent internal compiler errors on travis probably due to
low resources. The `platform_dependent` job works more consistently
but is roughly 10 minutes slower although it varies.
2. Remove status_checked jobs, as we are relying on CircleCI for
these now and do not really need platform coverage on them.
Fixes https://github.com/facebook/rocksdb/issues/9524
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9631
Test Plan: CI
Reviewed By: ajkr
Differential Revision: D34553989
Pulled By: pdillinger
fbshipit-source-id: a6e3a7276446721c4c0bebc4ed217c2ca2b53f11
3edbeeaa50