powerpc/syscalls: switch to generic syscallhdr.sh

Many architectures duplicate similar shell scripts.

This commit converts powerpc to use scripts/syscallhdr.sh.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210301153019.362742-2-masahiroy@kernel.org