Use enum for return method for dummy calls

Use enum for return method for dummy calls

In call_function_by_hand_dummy, struct_return and hidden_first_param_p
are used to represent a single concept. Replace with an enum.

gdb/ChangeLog:

	* gdbarch.sh (enum function_call_return_method): Add enum.
	* gdbarch.h: Regenerate.
	* infcall.c (call_function_by_hand_dummy): Replace vars with enum.