[POWERPC] Change ppc_rtas declaration to weak

Change the definition of powerpc's cond_syscall() to use the standard gcc
weak attribute specifier which provides proper support for C linkage as
needed by spu_syscall_table[].

Fixes this powerpc build error with CONFIG_SPU_FS=y, CONFIG_PPC_RTAS=n:

 arch/powerpc/platforms/built-in.o: undefined reference to `ppc_rtas'

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>