powerpc: Only build restart_table.c for 64s

Commit 9b69d48c ("powerpc/64e: remove implicit soft-masking and
interrupt exit restart logic") limited the implicit soft masking and
restart logic to 64-bit Book3S only. However we are still building
restart_table.c for all 64-bit, ie. Book3E also.

There's no need to build it for 64e, and it also causes missing
prototype warnings for 64e builds, because the prototype is already
behind an #ifdef PPC_BOOK3S_64.

Fixes: 9b69d48c

 ("powerpc/64e: remove implicit soft-masking and interrupt exit restart logic")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210701125026.292224-1-mpe@ellerman.id.au