Refactor arm_return_in_memory

Refactor arm_return_in_memory

Current arm_return_in_memory isn't friendly to adding new things in it.
Moreover, a lot of stuff are about APCS, which is not used nowadays (AAPCS
is being used).  This patch is to refactor arm_return_in_memory, so that
some code can be shared for both APCS and AAPCS at the beginning of
arm_return_in_memory, and then each ABI (APCS and AAPCS) are processed
separately.

gdb:

2015-11-13  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (arm_return_in_memory): Rewrite it.
	(arm_return_value): Call arm_return_in_memory for
	TYPE_CODE_COMPLEX.