x86: kprobes remove fix_riprel #ifdef

Move #ifdef around function definiton into the function and
unconditionally return on X86_32.  Saves an ifdef from the
one callsite.

[ mingo@elte.hu: minor cleanup. ]

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