Simplify {supply,fill}_{g,fp}regset

{supply,fill}_{g,fp}regset methods need only to access reg_buffer, instead
of regcache, so this patch change parameter type to reg_buffer.

Only works for aarch64 native

gdb:

2017-10-02  Yao Qi  <yao.qi@linaro.org>
:
	* aarch64-linux-nat.c (aarch64_linux_store_inferior_registers):
	(fill_gregset): -
	(fill_gregset): +
	(supply_gregset): -
	(supply_gregset): +
	(supply_gregset):
	(fill_fpregset): -
	(fill_fpregset): +
	(supply_fpregset): -
	(supply_fpregset): +
	* gregset.h (struct regcache;):