[zircon][kernel] kernel counters

Implementation based on the requirement doc
by travisg. This is only the core kernel part.

The implementation uses linker tricks to set
as much as possible static memory so they can
be used early and be declared almost anywhere.

The counters are stored in per-cpu arrays so
that there is no need to use atomics to keep
the increments consistent and there is no
false cache line sharing.

The counters are exposed via a temporary kernel
command 'k counters'

This change also adds 11 counters, here is sample
output.

$ k counters all
[00006.351] 01044.01047> 11 counters available:
[00006.351] 01044.01047> [0] kernel.dispatcher.cookie.reset <0>
[00006.351] 01044.01047> [1] kernel.dispatcher.cookie.set <0>
[00006.351] 01044.01047> [2] kernel.dispatcher.create
[00006.351] 01044.01047> [0:451][1:226][2:285][3:561] = 1523
[00006.351] 01044.01047> [3] kernel.dispatcher.destroy
[00006.351] 01044.01047> [0:223][1:174][2:161][3:354] = 912
[00006.351] 01044.01047> [4] kernel.dispatcher.observer.add
[00006.351] 01044.01047> [5] kernel.dispatcher.observer.cancel.byhandle
[00006.351] 01044.01047> [0:474][1:327][2:334][3:622] = 1757
[00006.351] 01044.01047> [6] kernel.dispatcher.observer.cancel.bykey <0>
[00006.351] 01044.01047> [7] kernel.thread.create
[00006.351] 01044.01047> [0:15][1:4][2:5][3:17] = 41
[00006.351] 01044.01047> [8] kernel.thread.join
[00006.351] 01044.01047> [1:1][2:4] = 5
[00006.351] 01044.01047> [9] kernel.thread.resume
[00006.351] 01044.01047> [0:12][1:4][2:5][3:17] = 38
[00006.351] 01044.01047> [10] kernel.thread.suspend <0>

Change-Id: I5eaabc3b7646dcaca72604953f0fb5338b3a776d