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.