x86_64: Implement idle mwait hint auto tuning

x86_64: Implement idle mwait hint auto tuning

The tuning is based on gd_idle_repeat.  The higher the gd_idle_repeat,
the deeper cpu specific C-state the current CPU will enter.

The idle mwait hint auto tuning could be enabled by setting sysctl
machdep.mwait.CX.idle to "AUTO" or "AUTODEEP":
"AUTO"     -- C0 ~ C2
"AUTODEEP" -- C0 ~ deepest cpu specific C-state

This will allow us to enter deepest cpu specific C-state, even if ACPI
does not map its deepest C-State to the deepest cpu specific C-state.

NOTE:
AUTODEEP and C3+ cpu specific C-state is not bullet prove yet.  We need
to get the bus master operations information from ACPI.