powerpc/xmon: Fix compile error with PPC_8xx=y

Rearrange the code so that mode and badaddr are only defined when
they're used.

Also unsplit the string for easier grepping, and switch from CONFIG_8xx
which is deprecated to CONFIG_PPC_8xx.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>