gdb-remote: Centralize thread specific packet handling

Summary:
Before this, each function had a copy of the code which handled appending of the thread suffix to
the packet (or using $Hg instead). I have moved that code into a single function and made
everyone else use that. The function takes the partial packet as a StreamString rvalue reference,
to avoid a copy and to remind the users that the packet will have undeterminate contents after
the call.

This also fixes the incorrect formatting of the QRestoreRegisterState packet in case thread
suffix is not supported.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D23604

llvm-svn: 279040