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

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

If kernel supports PTRACE_GETREGSET, GDB uses PTRACE_{G,S}ETREGSET
to fetch and store GP registers.

gdb:

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

	* arm-linux-nat.c (fetch_register): Use PTRACE_GETREGSET.
	(fetch_regs): Likewise.
	(store_regs): Use PTRACE_SETREGSET.
	(store_register): Likewise.