x86: use wrmsrl in kprobes.c, step.c

Where x86_32 passed zero in the high 32 bits, use wrmsrl which
will zero extend for us.  This allows ifdefs for 32/64 bit to
be eliminated.

Eliminate ifdef in step.c.  Similar cleanup was done when unifying
kprobes_32|64.c and wrmsr() was chosen there over wrmsrl().  This
patch changes these to wrmsrl.

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>