powerpc/process: Replace an #if defined(CONFIG_4xx) || defined(CONFIG_BOOKE) by IS_ENABLED()

The #if defined(CONFIG_4xx) || defined(CONFIG_BOOKE) encloses some
printk which can be compiled in all cases.

Replace by IS_ENABLED().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/a1b6ef3d657c8f249193442f56868fc358ea5b6c.1597643160.git.christophe.leroy@csgroup.eu