Add '+' sign at the end of echo message for debugserver

Summary:
-ammend to r238538
-Fix test failure of TestGdbRemoteAuxvSupport.py and TestLldbGdbServer.py on MacOSX

Test Plan:
./dotest -p TestGdbRemoteAuxvSupport.py
./dotest -p TestLldbGdbServer.py

Reviewers: chaoren, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D10133

llvm-svn: 238595