ftrace, powerpc, sparc64, x86: remove notrace from arch ftrace file

The entire file of ftrace.c in the arch code needs to be marked
as notrace. It is much cleaner to do this from the Makefile with
CFLAGS_REMOVE_ftrace.o.

[ powerpc already had this in its Makefile. ]

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>