powerpc: use always-y instead of extra-y in Makefiles

The extra-y syntax is planned for deprecation because it is similar
to always-y.

When building the boot wrapper, always-y and extra-y are equivalent.
Use always-y instead.

In arch/powerpc/kernel/Makefile, I added ifdef KBUILD_BUILTIN to
keep the current behavior: prom_init_check is skipped when building
only modular objects.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250602163302.478765-1-masahiroy@kernel.org