Fetch and store VFP registers by PTRACE_{G,S}ETREGSET

Fetch and store VFP registers by PTRACE_{G,S}ETREGSET

This patch is to use PTRACE_{G,S}ETREGSET to fetch and store VFP
registers if kernel supports.

gdb:

2015-06-01  Yao Qi  <yao.qi@linaro.org>

	* arm-linux-nat.c (fetch_vfp_regs): Use PTRACE_GETREGSET.
	(store_vfp_regs): Use PTRACE_SETREGSET.