Fix problem where "process detach" was not working properly. The

ptrace thread update that was replying to the SIGSTOP was also causing the
process to not really be sigstop'd any more so then the call to ptrace
detach was failing, and when debugserver exited the attached process
was being killed.  Now the ptrace thread update does not disturb the sigstop
state of the thread, so the detach works properly.

llvm-svn: 118018