Fix tag_ctl register size in the core file.

Fix tag_ctl register size in the core file.

This register should be 64 bits in size, but the current code only saves
32 bits. This is due to an early assumption that tag_ctl would be 32 bits
in size.

gdb/ChangeLog:

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

	* aarch64-linux-tdep.c
	(aarch64_linux_iterate_over_regset_sections): Update tag_ctl register
	size.
	* aarch64-linux-tdep.h (AARCH64_LINUX_SIZEOF_MTE_REGSET): Set to
	8 and update comments.