CPU back-end: limit the threads with a sub-device instead of a PoCL extension

[maleadt]

#821 limits the CPU back-end to as many PoCL threads as Julia has, by calling clSetCPUMaxComputeUnitsPOCL, an extension we carried in our PoCL builds. Upstream preferred making sub-devices usable over adding a global setting (pocl/pocl#2371), so the extension is gone again in pocl_standalone_jll 7.2.2, and this PR uses a sub-device instead.

A sub-device used to be just as slow as the whole device, because every command woke up all of PoCL's worker threads. pocl/pocl#2371 now only wakes the workers a command can use, on the sub-device it targets, and 7.2.2 includes that. So a sub-device sized to Julia's thread count behaves like the smaller thread pool the extension gave us. PoCL still starts a thread per hardware thread, but the ones outside the sub-device stay asleep and don't use CPU time.

Median saxpy launch time with -t4 on a 16-core/32-thread Ryzen 9950X:

	1k elements	64k	1M
extension (main, 7.2.1)	5 µs	9 µs	27 µs
sub-device (this PR, 7.2.2)	3 µs	10 µs	23–27 µs
sub-device with 7.2.1	31 µs	38 µs	55 µs

The behavior is the same as in #821, with one exception: JULIA_KA_CPU_THREADS can no longer go above the number of hardware threads, since a sub-device can't be larger than its device. PoCL's own variables still can, as the CPU docstring now says. If the device can't be partitioned (PoCL built with OpenMP, or the cpu-tbb device), the whole device is used.

The precompilation workload sets PoCL's thread-count variables again, as before #821, so that precompiling doesn't start a thread per core in every process.

This requires pocl_standalone_jll 7.2.2 (JuliaPackaging/Yggdrasil#15005), so CI will fail until that's registered.

The full test suite passes with -t4 against a local build of the 7.2.2 patch series.

[codecov]

Codecov Report

❌ Patch coverage is 93.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.33%. Comparing base (952c482) to head (8dda43c).

Files with missing lines	Patch %	Lines
src/pocl/nanoOpenCL.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #841      +/-   ##
==========================================
- Coverage   79.34%   79.33%   -0.01%     
==========================================
  Files          24       24              
  Lines        2091     2095       +4     
==========================================
+ Hits         1659     1662       +3     
- Misses        432      433       +1     

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