powerpc/syscalls: switch to generic syscalltbl.sh

Many architectures duplicate similar shell scripts.

This commit converts powerpc to use scripts/syscalltbl.sh. This also
unifies syscall_table_32.h and syscall_table_c32.h.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210301153019.362742-1-masahiroy@kernel.org