[POWERPC] Oprofile enhanced instruction sampling support

Oprofile enhanced instruction sampling support.

When performing instruction sampling, the mmcra[SLOT] field can be used to
more accurately identify the address of the sampled instruction.

Tested on power4, js20, power5 and power5+.

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