gdb: add missing regcache_map_entry array null terminators in aarch64-linux-tdep.c

Fix two spots in aarch64-linux-tdep.c that build regcache_map_entry
arrays without a null terminator.  The null terminators are needed for
regcache::transfer_regset and regcache_map_entry_size to work properly.

Change-Id: I3224a314e1360b319438f32de8c81e70ab42e105
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Luis Machado <luis.machado@arm.com>