Fix FFR register size for core files.

Fix FFR register size for core files.

The FFR register has a size of VL bits, not 32 bits.

This causes issues when writing core files with the gcore command and when
reading them. The FFR register sometimes shows up with garbage data.

gdb/ChangeLog:

2021-06-28  Luis Machado  <luis.machado@linaro.org>

	* aarch64-linux-tdep.c
	(aarch64_linux_iterate_over_regset_sections): Fix FFR register size.