gdbrunner: strip the newline off the result of "which executable"

Test: gdbclient.py -r echo foo
Change-Id: Id95d9f4e78c5e9dbd8ca8e30ffcfaff9bf5ef7c7
diff --git a/python-packages/gdbrunner/__init__.py b/python-packages/gdbrunner/__init__.py
index 8dff373..3063c75 100644
--- a/python-packages/gdbrunner/__init__.py
+++ b/python-packages/gdbrunner/__init__.py
@@ -267,7 +267,7 @@
 
     try:
         output, _ = device.shell(cmd)
-        return output
+        return adb.split_lines(output)[0]
     except adb.ShellError:
         raise  RuntimeError("Could not find executable '{}' on "
                             "device".format(executable_name))