[Driver] Add ClangFlags::TargetSpecific to simplify err_drv_unsupported_opt_for_target processing

clang/lib/Driver/ToolChains/Clang.cpp has a lot of fragments like the following:
```
if (const Arg *A = Args.getLastArg(...)) {
  if (Triple is xxx)
    A->render(Args, CmdArgs);
  else
    D.Diag(diag::err_drv_unsupported_opt_for_target) << ...;
}
```

The problem is more apparent with a recent surge of AIX-specific options.

Introduce the TargetSpecific flag so that we can move the target-specific
options to ToolChains/*.cpp and ToolChains/Arch/*.cpp and overload the
warn_drv_unused_argument mechanism to give an err_drv_unsupported_opt_for_target
error.

Migrate -march=/-mcpu= and some AIX-specific options to use this simplified pattern.

Reviewed By: jansvoboda11

Differential Revision: https://reviews.llvm.org/D151590