[gdb/symtab] Use task size in parallel_for_each in dwarf2_build_psymtabs_hard

[gdb/symtab] Use task size in parallel_for_each in dwarf2_build_psymtabs_hard

In dwarf2_build_psymtabs_hard, we use a parallel_for_each to distribute CUs
over threads.

Ensuring a fair distribution over the worker threads and main thread in terms
of number of CUs might not be the most efficient way, given that CUs can vary
in size.

Fix this by using per_cu->get_length () as the task size.

I've used this experiment to verify the performance impact:
...
$ for n in $(seq 1 10); do \
    time gdb -q -batch ~/firefox/libxul.so-93.0-1.1.x86_64.debug \
    2>&1 \
    | grep "real:"; \
  done
...
and without the patch got:
...
real: 4.71
real: 4.88
real: 4.29
real: 4.30
real: 4.65
real: 4.27
real: 4.27
real: 4.27
real: 4.75
real: 4.41
...
and with the patch:
...
real: 3.68
real: 3.81
real: 3.80
real: 3.68
real: 3.75
real: 3.69
real: 3.69
real: 3.74
real: 3.67
real: 3.74
...
so that seems a reasonable improvement.

With parallel_for_each_debug set to true, we get some more detail about
the difference in behaviour.  Without the patch we have:
...
Parallel for: n_elements: 2818
Parallel for: minimum elements per thread: 1
Parallel for: elts_per_thread: 704
Parallel for: elements on worker thread 0       : 705
Parallel for: elements on worker thread 1       : 705
Parallel for: elements on worker thread 2       : 704
Parallel for: elements on worker thread 3       : 0
Parallel for: elements on main thread           : 704
...
and with the patch:
...
Parallel for: n_elements: 2818
Parallel for: total_size: 1483674865
Parallel for: size_per_thread: 370918716
Parallel for: elements on worker thread 0       : 752   (size: 371811790)
Parallel for: elements on worker thread 1       : 360   (size: 371509370)
Parallel for: elements on worker thread 2       : 1130  (size: 372681710)
Parallel for: elements on worker thread 3       : 0     (size: 0)
Parallel for: elements on main thread           : 576   (size: 367671995)
...

Tested on x86_64-linux.