Fix "make check" on exotic architectures

With recent changes of applying clang format to WiredTiger code base (WT-4658), the wiredtiger-test-spinlock started to fail on zSeries machine, because of the triggering of s_clang_format script by make check, and the inexecutable clang_format binary built for x86 platform. 

http://build.wiredtiger.com:8080/job/wiredtiger-test-spinlock/4942/console

Making check in .

make[1]: Entering directory `/mnt/workspace/wiredtiger-test-spinlock/build_posix'

./s_clang-format: line 26: clang-format: command not found

--2019-08-21 00:08:19--  https://s3.amazonaws.com/boxes.10gen.com/build/clang-format-3.8-rhel55.tar.gz

(...) 

./s_clang-format: line 50: /mnt/workspace/wiredtiger-test-spinlock/dist/clang-format: cannot execute binary file

cat: write error: Broken pipe

Currently in s_clang_foramt there's checking/handling for different OS types, but lacks of a handling at platform/architecture level. The calling of s_clang_format script by "make check" is also interesting and should be investigated and understood.