gdbserver: add thread -> process backlink

In a few spots, we need to get to a process from a thread.  Having a
backlink from thread to process is cheap and makes the operation
trivial, add it.

Change-Id: I8a94b2919494b1dcaf954de2246386794308aa82
Reviewed-By: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>