xtensa: gcc: fix builtin_apply return value

xtensa: gcc: fix builtin_apply return value

xtensa may use up to 4 registers to return a value from a function, but
recognition of only one register in the xtensa_function_value_regno_p
and missing untyped_call pattern result in that only one register is
saved by the __builtin_apply and returned by the __builtin_apply_return.

gcc/
	* config/xtensa/xtensa.cc (xtensa_function_value_regno_p):
	Recognize all 4 return registers.
	* config/xtensa/xtensa.h (GP_RETURN_REG_COUNT): New definition.
	* config/xtensa/xtensa.md (untyped_call): New pattern.