[POWERPC] Emulate power5 popcntb instruction

In an attempt to make it easier for a power5 optimized app to run on a
power4 or a 970 or random earlier machine, this provides emulation of
the popcntb instruction.

Signed-off-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>