powerpc: Use vsprintf extention %pf with builtin_return_address

Emit the function name not the address when possible.

builtin_return_address() gives an address.  When building
a kernel with CONFIG_KALLSYMS, emit the actual function
name not the address.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>