aarch64: make HWCAP updates less error prone

Remove unused _DL_HWCAP_LAST definition and move _DL_HWCAP_COUNT
where it is needed (dl-procinfo.h always includes dl-procinfo.c).

	* sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
	(_DL_HWCAP_LAST): Remove.
	(_DL_HWCAP_COUNT): Move to ...
	* sysdeps/unix/sysv/linux/aarch64/dl-procinfo.c
	(_DL_HWCAP_COUNT): ... here.