powerpc: arch_has_single_step

This defines the new standard arch_has_single_step macro.  It makes the
existing set_single_step and clear_single_step entry points global, and
renames them to the new standard names user_enable_single_step and
user_disable_single_step, respectively.

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