[POWERPC] Cleanup asm-offsets.c

* Removed TI_EXECDOMAIN define as its not used anywhere
* Use STACK_INT_FRAME_SIZE to allow common define of INT_FRAME_SIZE
* Define TI_CPU on both ppc32 & ppc64 (removes an ifdef).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>