powerpc: Fix compile errors in prom_init_check for gcc 4.5

Just whitelist these extra compiler generated symbols.
Fixes these errors:

Error: External symbol '_restgpr0_14' referenced from prom_init.c
Error: External symbol '_restgpr0_20' referenced from prom_init.c
Error: External symbol '_restgpr0_22' referenced from prom_init.c
Error: External symbol '_restgpr0_24' referenced from prom_init.c
Error: External symbol '_restgpr0_25' referenced from prom_init.c
Error: External symbol '_restgpr0_26' referenced from prom_init.c
Error: External symbol '_restgpr0_27' referenced from prom_init.c
Error: External symbol '_restgpr0_28' referenced from prom_init.c
Error: External symbol '_restgpr0_29' referenced from prom_init.c
Error: External symbol '_restgpr0_31' referenced from prom_init.c
Error: External symbol '_savegpr0_14' referenced from prom_init.c
Error: External symbol '_savegpr0_20' referenced from prom_init.c
Error: External symbol '_savegpr0_22' referenced from prom_init.c
Error: External symbol '_savegpr0_24' referenced from prom_init.c
Error: External symbol '_savegpr0_25' referenced from prom_init.c
Error: External symbol '_savegpr0_26' referenced from prom_init.c
Error: External symbol '_savegpr0_27' referenced from prom_init.c
Error: External symbol '_savegpr0_28' referenced from prom_init.c
Error: External symbol '_savegpr0_29' referenced from prom_init.c
Error: External symbol '_savegpr0_31' referenced from prom_init.c

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>