[CI][C++] Travis s390x CI job is failing and blocks endianness related code verification

[asfimport]

Possibly due to travis memory limit, though only -j4
https://app.travis-ci.com/github/apache/arrow/jobs/531277678#L1132

Reporter: Yibo Cai / @cyb70289
Assignee: Kazuaki Ishizaki / @kiszk

PRs and other links:

• GitHub Pull Request #10952

_{Note: This issue was originally created as ARROW-13630. Please see the migration documentation for further details.}

[asfimport]

Kazuaki Ishizaki / @kiszk:
Thank you for creating an issue. Let us try to reduce -j at first.

[asfimport]

Yibo Cai / @cyb70289:
From https://docs.travis-ci.com/user/reference/overview/, there are only 2 cores available. Guess we can set it to -j2.

[asfimport]

Kazuaki Ishizaki / @kiszk:
Created a PR to reduce parallelism at #10952. Let us see what will happen

[asfimport]

Yibo Cai / @cyb70289:
Issue resolved by pull request 10952
#10952