Don't unconditionally set the sub group size.

[maleadt]

intel_reqd_sub_group_size is currently unconditionally set to the device's
reported subgroup size (or a heuristic default). However, the spec mentions:

Note that there is no guarantee for the value of get_sub_group_size()
even when this attribute is present, particularly when the work-group size
is not evenly divisible by the required sub-group size.

Specifically, PoCL reports a subgroup count of 0 when using a work-group size
that's smaller than the chosen subgroup size:

julia> @opencl kernel();
get_num_sub_groups() = 1

julia> @opencl sub_group_size=32 kernel();
get_num_sub_groups() = 0

The above is with the fix from this PR already, which only sets the
attribute when explicitly requesting a subgroup size. Normally, PoCL
determines an appropriate subgroup size per launch, so revert to that
by not setting the attribute by default.

This bug broke the RNG, which queries the sub group count. FWIW, this only
surfaced on JuliaGPU/GPUCompiler.jl#812, because
previously the exception trap was simply removed by PoCL resulting in
the subsequent memory access simply happening as if there was no OOB.

x-ref #413 (cc @christiangnrd)
x-ref https://registry.khronos.org/OpenCL/extensions/intel/cl_intel_required_subgroup_size.html
x-ref pocl/pocl#2181

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.03%. Comparing base (67580be) to head (7f1fed8).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/compiler/compilation.jl	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #436      +/-   ##
==========================================
- Coverage   79.19%   79.03%   -0.17%     
==========================================
  Files          12       12              
  Lines         745      744       -1     
==========================================
- Hits          590      588       -2     
- Misses        155      156       +1     

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