[ARM] Fix endless recursion on calculating CPRC candidate

When GDB determines whether type T can be part of candidate for
passing and returning in VFP registers, it calls
arm_vfp_cprc_sub_candidate recursively.  However, if type T has
self-reference field, like,

class C
{
  static C s;
};

arm_vfp_cprc_sub_candidate won't return.  This fix is to skip calling
arm_vfp_cprc_sub_candidate if the field is static.

gdb:

2016-07-06  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (arm_vfp_cprc_sub_candidate): Don't call
	arm_vfp_cprc_sub_candidate for static field.