hwpmc(9): Make pmclog buffer pcpu and update constants

On non-trivial SMP systems the contention on the pmc_owner mutex leads
to a substantial number of samples captured being from the pmc process
itself. This change a) makes buffers larger to avoid contention on the
global list b) makes the working sample buffer per cpu.

Run pmcstat in the background (default event rate of 64k):
pmcstat -S UNHALTED_CORE_CYCLES -O /dev/null sleep 600 &

Before:
make -j96 buildkernel -s >&/dev/null 3336.68s user 24684.10s system 7442% cpu 6:16.50 total

After:
make -j96 buildkernel -s >&/dev/null 2697.82s user 1347.35s system 6058% cpu 1:06.77 total

For more realistic overhead measurement set the sample rate for ~2khz
on a 2.1Ghz processor:
pmcstat -n 1050000 -S UNHALTED_CORE_CYCLES -O /dev/null sleep 6000 &

Collecting 10 samples of make -j96 buildkernel from each:

x before
+ after

real time:

N           Min           Max        Median           Avg        Stddev

x 10 76.4 127.62 84.845 88.577 15.100031
+ 10 59.71 60.79 60.135 60.179 0.29957192
Difference at 95.0% confidence

-28.398 +/- 10.0344
-32.0602% +/- 7.69825%
(Student's t, pooled s = 10.6794)

system time:

N           Min           Max        Median           Avg        Stddev

x 10 2277.96 6948.53 2949.47 3341.492 1385.2677
+ 10 1038.7 1081.06 1070.555 1064.017 15.85404
Difference at 95.0% confidence

-2277.47 +/- 920.425
-68.1574% +/- 8.77623%
(Student's t, pooled s = 979.596)

x no pmc
+ pmc running
real time:

HEAD:

N           Min           Max        Median           Avg        Stddev

x 10 58.38 59.15 58.86 58.847 0.22504567
+ 10 76.4 127.62 84.845 88.577 15.100031
Difference at 95.0% confidence

29.73 +/- 10.0335
50.5208% +/- 17.0525%
(Student's t, pooled s = 10.6785)

patched:

N           Min           Max        Median           Avg        Stddev

x 10 58.38 59.15 58.86 58.847 0.22504567
+ 10 59.71 60.79 60.135 60.179 0.29957192
Difference at 95.0% confidence

1.332 +/- 0.248939
2.2635% +/- 0.426506%
(Student's t, pooled s = 0.264942)

system time:

HEAD:

N           Min           Max        Median           Avg        Stddev

x 10 1010.15 1073.31 1025.465 1031.524 18.135705
+ 10 2277.96 6948.53 2949.47 3341.492 1385.2677
Difference at 95.0% confidence

2309.97 +/- 920.443
223.937% +/- 89.3039%
(Student's t, pooled s = 979.616)

patched:

N           Min           Max        Median           Avg        Stddev

x 10 1010.15 1073.31 1025.465 1031.524 18.135705
+ 10 1038.7 1081.06 1070.555 1064.017 15.85404
Difference at 95.0% confidence

32.493 +/- 16.0042
3.15% +/- 1.5794%
(Student's t, pooled s = 17.0331)

Reviewed by: jeff@
Approved by: sbruno@
Differential Revision: https://reviews.freebsd.org/D15155