[PowerPC] Disable regsets using zero sizes in gdbserver

[PowerPC] Disable regsets using zero sizes in gdbserver

Currently the linux-ppc-low.c fill/store functions for extended
regsets check whether they should execute by using the global hwcap
variable.

This patch explicitly sets the regset sizes to zero when needed to
disable them instead, so that the fill/store functions are not called
in the first place by regsets_fetch_inferior_registers in linux-low.c.

gdb/gdbserver/ChangeLog:
2018-05-22  Pedro Franco de Carvalho  <pedromfc@linux.vnet.ibm.com>

	* linux-ppc-low.c (ppc_fill_vsxregset): Remove ppc_hwcap check.
	(ppc_store_vsxregset): Likewise.
	(ppc_fill_vrregset): Likewise.
	(ppc_store_vrregset): Likewise.
	(ppc_fill_evrregset): Likewise.
	(ppc_store_evrregset): Likewise.
	(ppc_regsets): Set VSX/VR/EVR regset sizes to 0.
	(ppc_arch_setup): Iterate through ppc_regsets and set sizes when
	needed.