ptrace_clear_single_step

[David Xu]

Is there any reason that ptrace_clear_single_step is not needed ?
I found that alpha implemented it, but not other architectures.
For multiple threads, debugger need to turn off single step status for
specific thread, I found gdb even hacked it to clear single step flag
in %eflag registers on i386 because kernel does not provide a clean
way to clear it.

David Xu