switch SPIR-V backend from LLVM to Khronos

[simeonschaub]

As discussed in JuliaGPU/OpenCL.jl#353 (comment)

[maleadt]

That doesn't need a change here, OpenCL.jl can simply pass a different backend flag here: https://github.com/JuliaGPU/OpenCL.jl/blob/7a7b8e92c95717b65e0696b0c0f6205131b29e4a/src/compiler/compilation.jl#L52

That said, with #709 being uncertain (see #709 (comment)) the change isn't strictly necessary anymore. I'm not sure yet how we'll get out of this yet, maybe it's best to switch PoCL over to CBS for now (pocl/pocl#1974) and use arm_terminate_kernel once PoCL supports cl_arm_controlled_kernel_termination.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.33%. Comparing base (32b4fc8) to head (b230a7e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #710      +/-   ##
==========================================
+ Coverage   75.27%   75.33%   +0.05%     
==========================================
  Files          24       24              
  Lines        3523     3523              
==========================================
+ Hits         2652     2654       +2     
+ Misses        871      869       -2     

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[simeonschaub]

I was more concerned with JuliaGPU/OpenCL.jl#353 here, but I see your point