gdb: fix libstdc++ assert caused by invalid use of std::clamp

After this commit:

  commit 33ae4543
  Date:   Mon Dec 4 14:23:17 2023 +0000

      gdb: Enable early init of thread pool size

I am now seeing this assert from libstdc++:

  /usr/include/c++/9/bits/stl_algo.h:3715: constexpr const _Tp& std::clamp(const _Tp&, const _Tp&, const _Tp&) [with _Tp = int]: Assertion '!(__hi < __lo)' failed.

This may only be visible because I compile with:

  -D_GLIBCXX_DEBUG=1 -D_GLIBCXX_DEBUG_PEDANTIC=1

but I haven't checked.  The issue the assert is highlighting is real,
and is caused by this block of code:

  if (n_threads < 0)
    {
      const int hardware_threads = std::thread::hardware_concurrency ();
      /* Testing in #29959 indicates that parallel efficiency drops between
         n_threads=5 to 8.  Therefore, clamp the default value to 8 to avoid an
         excessive number of threads in the pool on many-core systems.  */
      const int throttle = 8;
      n_threads = std::clamp (hardware_threads, hardware_threads, throttle);
    }

The arguments to std::clamp are VALUE, LOW, HIGH, but in the above, if
we have more than 8 hardware threads available the LOW will be greater
than the HIGH, which is triggering the assert I see above.

I believe std::clamp is the wrong tool to use here.  Instead std::min
would be a better choice; we want the smaller value of
HARDWARE_THREADS or THROTTLE.  If h/w threads is 2, then we want 2,
but if h/w threads is 16 we want 8, this is what std::min gives us.

After this commit, I no longer see the assert.