Move ARM_CPSR_GREGNUM to arch/arm-linux.h

Move ARM_CPSR_GREGNUM to arch/arm-linux.h

This patch moves macro ARM_CPSR_GREGNUM to arch/arm-linux.h so that it
can be used in GDBserver side.

gdb:

2016-04-20  Yao Qi  <yao.qi@linaro.org>

	* arm-linux-tdep.h (ARM_CPSR_GREGNUM): Move it to ...
	* arch/arm-linux.h: ... here.

gdb/gdbserver:

2016-04-20  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch32-low.c: Include "arch/arm-linux.h".
	(arm_fill_gregset): Use ARM_CPSR_GREGNUM rather than magic
	number 16.
	(arm_store_gregset): Likewise.