Only allow a process to use the x86 RDPMC instruction if it has
allocated and attached a PMC to itself.
Inform the MD layer of the "pseudo context switch out" that needs
to be done when the last thread of a process is exiting.
includes the MD header for us. Do not include <machine/specialreg.h>
as it is not a header file that can be included from MI files. It
is included from <machine/pmc_mdep.h> if so needed and possible.
Ok'd: jkoshy@