Improve PTX ISA selection

[maleadt]

Instead of hard-coding it to 6.3, select the highest version available. We now also differentiate between what LLVM supports, which is part of the CompilerTarget, and what the CUDA toolkit does, which we now store in the CUDACompilerParams. For the compute capability, that means emitting code for e.g. sm_89 and passing -arch=sm_90 to ptxas. For the PTX ISA, that's not possible, so we string-replace the .version directive in the generated assembly. Feels icky, but I think it should work (on the condition we don't use instructions that are deprecated between the PTX ISA used by LLVM, and the one we replace it with, but that's generally a very small window).

One annoying aspect is that the compute_version() and ptx_isa() getters for kernel code currently return the LLVM-level compatibility, so we might not generate the best code. However, I don't think we can bump this to the CUDA-level compatibility, as that may risk running into LLVM selection errors. And it doesn't seem worth splitting into llvm_compute_capability and cuda_compute_capability, where the latter can only use inline assembly.

Fixes #2080

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10% 🎉

Comparison is base (a0dfc35) 71.59% compared to head (ce6b8cc) 71.70%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2088      +/-   ##
==========================================
+ Coverage   71.59%   71.70%   +0.10%     
==========================================
  Files         157      157              
  Lines       13812    13834      +22     
==========================================
+ Hits         9889     9919      +30     
+ Misses       3923     3915       -8     

Files Changed	Coverage Δ
src/compiler/execution.jl	84.00% <ø> (ø)
src/utilities.jl	83.01% <ø> (-0.32%)	⬇️
src/compatibility.jl	98.14% <100.00%> (+1.85%)	⬆️
src/compiler/compilation.jl	95.37% <100.00%> (+5.44%)	⬆️

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