x86, kprobes: remove sparse warnings from x86

arch/x86/kernel/kprobes.c:584:16: warning: symbol 'kretprobe_trampoline_holder' was not declared. Should it be static?
arch/x86/kernel/kprobes.c:676:6: warning: symbol 'trampoline_handler' was not declared. Should it be static?

Make them static and add the __used attribute, approach taken from the
arm kprobes implementation.

kretprobe_trampoline_holder uses inline assemly to define the global
symbol kretprobe_trampoline, but nothing ever calls the holder explicitly.

trampoline handler is only called from inline assembly in the same file,
mark it used and static.

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Acked-by: Masami Hiramatsu <mhiramat@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>