Clang-specific flag is passed to assembler if -pgmc is overridden

46f9a476 passes the Clang-specific -Qunused-arguments flag to the cc and as command-line flags when it detects that pgmc is a Clang compiler. However, this breaks surprisingly if the user overrides cc with pgmc yet leaves pgma untouched. Ideally we would test as and cc individually and pass -Qunused-arguments only when appropriate.