[Morello] Prototype passing all memory arguments through C9

This prototypes https://github.com/ARM-software/abi-aa/issues/158

This currently doesn't support a large number of arguments, where
padding still needs to be added.

Some notable changes:
- The va_list is made read-only in the callee to enable writes
  to the memory pointed by C9.
- C9 is set to NULL in the caller if the callee is variadic and
  we don't have any arguments passed through memory.

This behavior can be enabled in clang by passing -Xclang -morello-bounded-memargs.
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