[POWERPC] Make cpus_in_xmon static and remove extern mess from hvc_console.c

This is a little messier than I'd like because xmon.h only exists
on powerpc and we can't have a static inline and an extern declaration
visible at the same time.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>