Merged ppc_asm.h between ppc32 & ppc64.  The majority of the file is
common between the two architectures excluding how a single GPR is
saved/restored and which GPRs are non-volatile.

Additionally, moved the ASM_CONST macro used on ppc64 into ppc_asm.h.
Signed-off-by: Kumar Gala <kumar.gala@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>