cache: avoid hardcoded zone alignment

Previously, this was underaligned on CHERI system where pointers are
larger than time_t.

Use the alignment of struct namecache_ts which picks up time_t via strut
timespec and pointers via struct namecache. This arguably overaligns
cache_zone_small and cache_zone_large on i386 kernels, but I suspect the
actual microarchitectures most i386 binaries are run on do better with
64-bit alignment.

Reviewed by: olce, markj
Effort: CHERI upstreaming
Sponsored by: Innovate UK
Fixes: cf8ac0de8150 ("cache: reduce zone alignment to 8 bytes")
Differential Revision: https://reviews.freebsd.org/D54376