gdb: Enable early init of thread pool size

gdb: Enable early init of thread pool size

This commit enables the early initialization commands (92e4e97a9f5) to
modify the number of threads used by gdb's thread pool.

The motivation here is to prevent gdb from spawning a detrimental number
of threads on many-core systems under environments with restrictive
ulimits.

With gdb before this commit, the thread pool takes the following sizes:

1. Thread pool size is initialized to 0.

2. After the maintenance commands are defined, the thread pool size is
set to the number of system cores (if it has not already been set).

3. Using early initialization commands, the thread pool size can be
changed using "maint set worker-threads".

4. After the first prompt, the thread pool size can be changed as in the
previous step.

Therefore after step 2. gdb has potentially launched hundreds of threads
on a many-core system.

After this change, step 2 and 3 are reversed so there is an opportunity
to set the required number of threads without needing to default to the
number of system cores first.

There does exist a configure option (added in 261b07488b9) to disable
multithreading, but this does not allow for an already deployed gdb to
be configured.

Additionally, the default number of worker threads is clamped at eight
to control the number of worker threads spawned on many-core systems.
This value was chosen as testing recorded on bugzilla issue 29959
indicates that parallel efficiency drops past this point.

GDB built with GCC 13.

No test suite regressions detected. Compilers: GCC, ACfL, Intel, Intel
LLVM, NVHPC; Platforms: x86_64, aarch64.

The scenario that interests me the most involves preventing GDB from
spawning any worker threads at all. This was tested by counting the
number of clones observed by strace:

    strace -e clone,clone3 gdb/gdb -q \
    --early-init-eval-command="maint set worker-threads 0" \
    -ex q ./gdb/gdb |& grep --count clone

The new test relies on "gdb: install CLI uiout while processing early
init files" developed by Andrew Burgess. This patch will need pushing
prior to this change.

The clamping was tested on machines with both 16 cores and a single
core.  "maint show worker-threads" correctly reported eight and one
respectively.

Approved-By: Tom Tromey <tom@tromey.com>