Fix signle stepping on arm when multiple thread is involved

On linux-arm we use software single stepping where setting the new
breakpoint is only possible while the process is in stopped state.
This CL moves the setup code for single stepping form the SigneStep
operation into the Resum method to avoid an error when the process
already started when we want to step one of the thread.

Differential revision: http://reviews.llvm.org/D9108

llvm-svn: 235494