MFC 278474,278476,280279:
Expand the bitcount*() API and use it to implement CPU_COUNT for cpusets.

278474:
Use __builtin_popcnt() to implement a BIT_COUNT() operation for bitsets and
use this to implement CPU_COUNT() to count the number of CPUs in a cpuset.

278476:
Use builtin_popcountl() instead of builtin_popcount().

280279:
Expand the bitcount* API to support 64-bit integers, plain ints and longs
and create a "hidden" API that can be used in other system headers without
adding namespace pollution.

• If the POPCNT instruction is enabled at compile time, use builtin_popcount*() to implement bitcount*(), otherwise fall back to software implementations.
• Use the existing bitcount16() and bitcount32() from <sys/systm.h> to implement the non-POPCNT bitcount16() and bitcount32() in <sys/types.h>.
• For the non-POPCNT bitcount64(), use a similar SWAR method on 64-bit systems. For 32-bit systems, use two bitcount32() operations on the two halves.
• Use bitcount32() to provide a bitcount() that operates on plain ints.
• Use either bitcount32() or bitcount64() to provide a __bitcountl() that operates on longs.
• Add public bitcount*() wrappers for __bitcount*() for use in the kernel in <sys/libkern.h>.
• Use bitcountl() instead of builtin_popcountl() in BIT_COUNT().