powerpc: Stack Pointer not properly aligned

The code first aligns the SP to 16 then subtract 8, making it
8 bytes aligned. Furthermore the initial stack frame not
quite correct either.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: Andy Fleming <afleming@freescale.com>