Fix group stop signal handling issue on Linux.

This patch addresses a bug where in a multi-threaded program a new
signal from the inferior may be received before all group-stop
messages from an earlier signal have been handled.

Patch by Andrew MacPherson


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@200226 91177308-0d34-0410-b5e6-96231b3b80d8