ftrace: build fix with gcc 4.3

fix:

arch/x86/kernel/ftrace.c: Assembler messages:
arch/x86/kernel/ftrace.c:82: Error: bad register name `%sil'
make[1]: *** [arch/x86/kernel/ftrace.o] Error 1

Signed-off-by: Ingo Molnar <mingo@elte.hu>