Fix a problem where lldb-mi would not stop the debuggee after -exec-interrupt command.

Summary:
This revision fixes a problem where lldb-mi would not stop the execution after exec-interrupt call.
On Linux, SIGSTOP is used to stop the debuggee process. LLDB stopped the debuggee alright. But when
lldb-mi received the notification of stopping with reason as SIGSTOP, it would resume the process.
This was heppening in CMICmnLLDBDebuggerHandleEvents::HandleProcessEventStopSignal. This function aslo
used hard coded numbers for signal istead of symbolic names.

This revision changes code to treat SIGSTOP reason as SIGINT. Also used symbolic names for signals
instead of numbers.

Reviewers: ki.stfu, clayborg

Reviewed By: ki.stfu, clayborg

Subscribers: zturner, lldb-commits

Differential Revision: http://reviews.llvm.org/D7783

llvm-svn: 230237