Use existing helper function to implement popcntd instruction

The recent patches adding partial support for POWER7 cpu emulation included
implementing the popcntd instruction.  The support for this was open coded,
but host-utils.h already included a function implementing an equivalent
population count function, which uses a gcc builtin (which can use special
host instructions) if available.

This patch makes the popcntd implementation use the existing, potentially
faster, implementation.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>