powerpc/machdep: warn when machine_is() used too early

machine_is() can't provide correct results before probe_machine() has
run. Warn when it's used too early in boot, placing the WARN_ON() in a
helper function so the reported file:line indicates exactly what went
wrong.

checkpatch complains about __attribute__((weak)) in the patch, so
change that to __weak, and align the line continuations as well.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230210-warn-on-machine-is-before-probe-machine-v2-1-b57f8243c51c@linux.ibm.com