Allow struct 'return' on 32-bit sparc.

Allow struct 'return' on 32-bit sparc.

gdb/

	* sparc-tdep.c (sparc32_return_value): Handle writing return value when
	using RETURN_VALUE_ABI_PRESERVES_ADDRESS.
	* value.c (struct_return_convention): New function.
	(using_struct_return): Implement in terms of struct_return_convention.
	* value.h (struct_return_convention): Declare.
	* stack.c (return_command): Allow successful overriding of the return
	value when RETURN_VALUE_ABI_PRESERVES_ADDRESS.