[Clang FE, SystemZ] Recognize -mpacked-stack CL option

Recognize -mpacked-stack from the command line and add a function attribute
"mpacked-stack" when passed. This is needed for building the Linux kernel.

If this option is passed for any other target than SystemZ, an error is
generated.

Review: Ulrich Weigand
https://reviews.llvm.org/D71441