POWERPC: use generic per cpu

Powerpc has a way to determine the address of the per cpu area of the
currently executing processor via the paca and the array of per cpu
offsets is avoided by looking up the per cpu area from the remote
paca's (copying x86_64).

Cc: Paul Mackerras <paulus@samba.org>
Cc: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Mike Travis <travis@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Olof Johansson <olof@lixom.net>
Tested-by: Geoff Levand <geoffrey.levand@am.sony.com>