malloc: provide 384 byte zone

Total page count after buildworld on ZFS for 384 (if present) and 512 zones:
before: 29713
after: 25946

per-zone page use:
vm.uma.malloc_384.keg.domain.1.pages: 11621
vm.uma.malloc_384.keg.domain.0.pages: 11597
vm.uma.malloc_512.keg.domain.1.pages: 1280
vm.uma.malloc_512.keg.domain.0.pages: 1448

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D27145