llgs: corrected Linux signal reception notification for SIGABRT, SIGSEGV and their ilk.

Added llgs/debugserver gdb-remote tests around SIGABRT and SIGSEGV signal reception
notification.  Found a few bugs in exception signal handling in Linux llgs.  Fixed those.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@215458 91177308-0d34-0410-b5e6-96231b3b80d8