x86_64: Export Cx support list through sysctl machdep.mwait.CX.supported

The format is: C-state/sub-state
x86_64: Add new sysctls to set idle/spin mwait hint

These sysctls verify that the mwait hint is supported.  It's format is:
C-state/sub-state.  The valid one could be picked up from the output of
sysctl machdep.mwait.CX.supported.

Sysctls hw.cpu_mwait_{halt,spin} should be deprecated.

While I'm here, rearrange cpu_mwait_hint_valid() a little bit.