powerpc/xmon: Exclude all of xmon from ftrace

Exclude core xmon files from ftrace (along with an xmon xive helper
outside of xmon/) to minimize impact of ftrace while within xmon.

Before:
  /sys/kernel/debug/tracing# grep -ci xmon available_filter_functions
  26

After:
  /sys/kernel/debug/tracing# grep -ci xmon available_filter_functions
  0

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
[mpe: Use $(subst ..) on KBUILD_CFLAGS rather than CFLAGS_REMOVE_xxx]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>