x86: kprobe-booster for x86-64

This patch adds kprobe-booster to kprobes_64.c.

- Changes are based on x86-32.
- Add REX prefix checking code.

Signed-off-by: Masami Hiramatsu <mhiramat@redhat.com>
Signed-off-by: Jim Keniston <jkenisto@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>