x86: single_step moved

This moves the single-step support code from ptrace_64.c into a new file
step.c, verbatim.  This paves the way for consolidating this code between
64-bit and 32-bit versions.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>