ARROW-13026: [CI] Use LLVM 10 for s390x
ARROW-13026: [CI] Use LLVM 10 for s390x

[Recent TravisCI for s390x C](https://travis-ci.com/github/apache/arrow/jobs/512489573#L586) causes the following error. The file should be downloaded `...20210605...`. To fix this issue, this PR tries to download LLVM from the default apt repository instead of the LLVM apt repository.

```
Err:8 https://apt.llvm.org/focal llvm-toolchain-focal-12/main s390x libllvm12 s390x 1:12.0.1~++20210604112550+6279fd114acb-1~exp1~20210604213327.98

  404  Not Found [IP: 199.232.38.49 443]
```

Closes #10525 from kiszk/ARROW-13026

Authored-by: Kazuaki Ishizaki <409019bab0e07036ad6abfa289bcfe098eb16317@jp.ibm.com>
Signed-off-by: Kazuaki Ishizaki <409019bab0e07036ad6abfa289bcfe098eb16317@jp.ibm.com>

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