MFC r362661: Add mostly dummy hw.pci.enable_aspm tunable.

The only thing this tunable enables now is reporting to ACPI _OSC that
Active State Power Management and Clock Power Management Capability are
"supported" by the OS.

I've found that at least some Supermicro server boards do not allow OS
to support native PCIe hot-plug unless it reports those capabilities.
After spending significant time in PCIe specs I have found very little
motivation for that, and none of it applies to those motherboards, not
enabling ASPM themselves. So unless OS explicitly wants to save power,
I see nothing for it to do there actually.

I guess it may get sense to support ASPM when we get Thunderbolt support.
Otherwise I have no system with PCIe hot-plug where power saving matters.

It would be nice to enable this by default, but I worry that it affect
power saving of some laptops, even though I haven't noticed that myself.