Use the cached dc zva length in arm64 memset

On boot we cache the length the 'dc zva' instruction will zero. Use
this in the memset function to decide when to use it. As the cached
value is in .bss it will be zero on boot so memset is safe to use
before the value has been read.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 7d90ce7cae63a8121da0acc3ce36a713a98d6033)