Added gdb-remote test for s packet, single stepping.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@210481 91177308-0d34-0410-b5e6-96231b3b80d8
Merge upstream llvm.org r210481 into dev-tfiala-native-protocol-linux-x86_64
Implemented s single step for llgs/linux-x86_64.