gdbserver/Linux: Introduce NULL_REGSET

gdbserver/Linux: Introduce NULL_REGSET

Fixes errors like:

 src/gdb/gdbserver/linux-x86-low.c:477:1: error: invalid conversion from 'int' to 'regset_type' [-fpermissive]

gdb/gdbserver/ChangeLog:
2015-10-29  Pedro Alves  <palves@redhat.com>

	* linux-low.h (NULL_REGSET): Define.
	* linux-aarch64-low.c (aarch64_regsets): Use NULL_REGSET.
	* linux-arm-low.c (arm_regsets): Likewise.
	* linux-crisv32-low.c (cris_regsets): Likewise.
	* linux-m68k-low.c (m68k_regsets): Likewise.
	* linux-mips-low.c (mips_regsets): Likewise.
	* linux-nios2-low.c (nios2_regsets): Likewise.
	* linux-ppc-low.c (ppc_regsets): Likewise.
	* linux-s390-low.c (s390_regsets): Likewise.
	* linux-sh-low.c (sh_regsets): Likewise.
	* linux-sparc-low.c (sparc_regsets): Likewise.
	* linux-tic6x-low.c (tic6x_regsets): Likewise.
	* linux-tile-low.c (tile_regsets): Likewise.
	* linux-x86-low.c (x86_regsets): Likewise.
	* linux-xtensa-low.c (xtensa_regsets): Likewise.