Re-apply r306516 (by cem):

Re-apply r306516 (by cem):

Reduce the cost of TLB invalidation on x86 by using per-CPU completion flags

Reduce contention during TLB invalidation operations by using a per-CPU
completion flag, rather than a single atomically-updated variable.

On a Westmere system (2 sockets x 4 cores x 1 threads), dtrace measurements
show that smp_tlb_shootdown is about 50% faster with this patch; observations
with VTune show that the percentage of time spent in invlrng_single_page on an
interrupt (actually doing invalidation, rather than synchronization) increases
from 31% with the old mechanism to 71% with the new one.  (Running a basic file
server workload.)

Submitted by:	Anton Rang <rang at acm.org>
Reviewed by:	cem (earlier version)
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D8041