gdb/py-inferior: Keep inferior threads in a map

gdb/py-inferior: Keep inferior threads in a map

The python code maintains a list of threads for each inferior.  This
list is implemented as a linked list.  When the number of threads grows
high, this implementation can begin to be a performance bottleneck as
finding a particular thread_object in the list has a complexity of O(N).

We see this in ROCgdb[1], a downstream port of GDB for AMDGUP.  On
AMDGPU devices, the number of threads can get significantly higher than
on usual GDB workloads.

In some situations, we can reach the end of the inferior process with
GDB still having a substantial list of known threads.  While running
target_mourn_inferior, we end up in inferior::clear_thread_list which
iterates over all remaining threads and marks each thread exited.  This
fires the gdb::observers::thread_exit observer and eventually
py-inferior.c:set_thread_exited gets called.  This function searches in
the linked list with poor performances.

This patch proposes to change the linked list that keeps the per
inferior_object list of thread_objects into a std::unordered_map.  This
allows to have the search operation complexity be O(1) on average
instead of O(N).

With this patch, we can complete clear_thread_list in about 2.5 seconds
compared to 10 minutes without it.

Except for the performance change, no user visible change is expected.

Regression tested on Ubuntu-22.04 x86_64.

[1] https://github.com/ROCm-Developer-Tools/ROCgdb