Merge #17470: ci: Use clang-8 for fuzzing to run on aarch64 ci systems

fa2ec9f451 fuzz: Bump timeout in test_runner to accomodate for slow arm64 CPUs (MarcoFalke)
fa6e01b2f3 ci: Use clang-8 for fuzzing to run on aarch64 ci systems (MarcoFalke)

Pull request description:

  Ubuntu bionic clang is clang version 6, which does not come with libfuzzer. So the ci system breaks down when run on aarch64.

  Fix that by using clang-8

  For reference, the previous error on my ci system was:

  ```
  /usr/bin/ld: cannot find /usr/lib/llvm-6.0/lib/clang/6.0.0/lib/linux/libclang_rt.fuzzer-aarch64.a: No such file or directory

ACKs for top commit:
  laanwj:
    ACK fa2ec9f451

Tree-SHA512: 4954dbc36c444d1ae145290115eea6291753c9810c92003ab8d75433c3fe3bfee439d3a99dc394418275527157a8b89f04038c8b16e08c69ec9ded50fb869e70

test/fuzz/test_runner.py

@@ -78,7 +78,7 @@ def main():
                 os.path.join(config["environment"]["BUILDDIR"], 'src', 'test', 'fuzz', test_list_selection[0]),
                 '-help=1',
             ],
-            timeout=1,
+            timeout=10,
             check=True,
             stderr=subprocess.PIPE,
             universal_newlines=True,