powerpc: Add ppc_function_entry() which gets the entry point for a function

Because function pointers point to different things on 32-bit vs 64-bit,
add a macro that deals with dereferencing the OPD on 64-bit.  The soon to
be merged ftrace wants this, as well as other code I am working on.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>