ARROW-13630: [CI][C++][s390x] Reduce parallelism to build Arrow library

Closes apache#10952 from kiszk/ARROW-13630

Authored-by: Kazuaki Ishizaki <[email protected]>
Signed-off-by: Yibo Cai <[email protected]>

Author: kiszk

.travis.yml

@@ -100,7 +100,7 @@ jobs:
           -e ARROW_PARQUET=OFF
           -e ARROW_S3=OFF
           -e CMAKE_UNITY_BUILD=ON
-          -e CPP_MAKE_PARALLELISM=4
+          -e CPP_MAKE_PARALLELISM=2
           -e PARQUET_BUILD_EXAMPLES=OFF
           -e PARQUET_BUILD_EXECUTABLES=OFF
           -e Protobuf_SOURCE=BUNDLED