RFC: hwpmc soft arm

[Fabien Thomas]

	Hi,

This patch add sampling support for arm using soft PMC.
At this time only with one level (no backtrace), i'm planning to add it
with ddb code as a base but if some arm guru can give me some pointer it will help.

The patch is tested on 8.3 but should works on head.

patch:
http://freebsd.org/~fabient/patch-hwpmc_arm_rev1

add to your kernel conf:
options         HWPMC_HOOKS
device          hwpmc # can be added as a module
options         DEVICE_POLLING

sample capture during buildworld:
kldload hwpmc # if build as a module
pmcstat -Sclock.hard -T -w4
http://freebsd.org/~fabient/pmc_arm.png

Fabien