Use regcache->tdesc instead of arm_hwcap

Use regcache->tdesc instead of arm_hwcap

arm_hwcap is a global variable, and we should avoid using it as much
as we can.  Instead of checking arm_hwcap, we can check whether
regcache->tdesc is a certain kind of target description.  This is
what this patch does.

gdb/gdbserver:

2015-07-30  Yao Qi  <yao.qi@linaro.org>

	* linux-arm-low.c (arm_fill_wmmxregset): Don't use arm_hwcap.
	Use regcache->tdesc instead.
	(arm_store_wmmxregset): Likewise.
	(arm_fill_vfpregset): Likewise.
	(arm_store_vfpregset): Likewise.