Add default VESPA_CPU_ARCH_FLAGS setting when using clang on Fedora.
Add default VESPA_CPU_ARCH_FLAGS setting when using clang on Fedora.

vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa,vespa-engine/vespa