[gdbsupport] Improve thread scheduling in parallel_for_each

When running a task using parallel_for_each, we get the following
distribution:
...
Parallel for: n_elements: 7271
Parallel for: minimum elements per thread: 10
Parallel for: elts_per_thread: 1817
Parallel for: elements on worker thread 0       : 1817
Parallel for: elements on worker thread 1       : 1817
Parallel for: elements on worker thread 2       : 1817
Parallel for: elements on worker thread 3       : 0
Parallel for: elements on main thread           : 1820
...

Note that there are 4 active threads, and scheduling elts_per_thread on each
of those handles 4 * 1817 == 7268, leaving 3 "left over" elements.

These leftovers are currently handled in the main thread.

That doesn't seem to matter much for this example, but for say 10 threads and
99 elements, you'd have 9 threads handling 9 elements and 1 thread handling 18
elements.

Instead, distribute the left over elements over the worker threads, such that
we have:
...
Parallel for: elements on worker thread 0       : 1818
Parallel for: elements on worker thread 1       : 1818
Parallel for: elements on worker thread 2       : 1818
Parallel for: elements on worker thread 3       : 0
Parallel for: elements on main thread           : 1817
...

Tested on x86_64-linux.