Basic ABI changes for passing structs by value in 64-bit Darwin.

    Basic ABI changes for passing structs by value in 64-bit Darwin.
        * config/rs6000/rs6000.c (rs6000_darwin64_abi): New flag.
        (rs6000_override_options): Set it for 64-bit Darwin.
        (rs6000_parse_abi_options): Add testing options to change it.
        (rs6000_return_in_memory): Test whether the type is one
        that can be passed in registers.
        (darwin64_function_arg_advance): New.
        (function_arg_advance): Call it, plus add recursion depth
        argument and test when counting off arguments.
        (rs6000_darwin64_function_arg): New.
        (function_arg): Call it.
        (setup_incoming_varargs): Add argument to function_arg_advance.
        (rs6000_darwin64_function_value): New.
        (rs6000_function_value): Call it.
        * config/rs6000/rs6000.h (FUNCTION_ARG_ADVANCE): Pass depth arg.
        * config/rs6000/rs6000-protos.h: Update decl of
        function_arg_advance.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@90629 138bc75d-0d04-0410-961f-82ee72b054a4