powerpc: Add asm/syscall.h with the tracehook entry points

Add asm/syscall.h for powerpc with all the required entry points.
This will allow arch-independent tracing code for system calls.

BenH: Fixed up use of regs->trap to properly mask low bit

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>