Add COUNTER_U64_SYSINIT() and COUNTER_U64_DEFINE_EARLY().

The aim is to reduce the boilerplate needed today to define and
initialize global counters. Also add SI_SUB_COUNTER to the sysinit
ordering.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23977