Ensure 'struct thread' is aligned to a cache line

Ensure 'struct thread' is aligned to a cache line

Using the new UMA_ALIGN_CACHE_AND_MASK() facility, which allows to
simultaneously guarantee a minimum of 32 bytes of alignment (the 5 lower
bits are always 0).

For the record, to this day, here's a (possibly non-exhaustive) list of
synchronization primitives using lower bits to store flags in pointers
to thread structures:
- lockmgr, rwlock and sx all use the 5 bits directly.
- rmlock indirectly relies on sx, so can use the 5 bits.
- mtx (non-spin) relies on the 3 lower bits.

Reviewed by:            markj, kib
MFC after:              2 week
Sponsored by:           The FreeBSD Foundation
Differential Revision:  https://reviews.freebsd.org/D42266