Add dwarf_version function so targets can choose

[gbaraldi]

It defaults to 4 like before but PTX sets it to 2 because that's what cuda supports. Following https://github.com/search?q=repo%3Allvm%2Fllvm-project%20GetDefaultDwarfVersion()&type=code

[vchuravy]

I am not sure where the CUDA supports V2 came from in my head, but looks like clang sets it for v5 for AMD so this is a good change nonetheless.

Great thank you!

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -7.83% ⚠️

Comparison is base (06e6706) 81.12% compared to head (2c4e9e1) 73.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
- Coverage   81.12%   73.29%   -7.83%     
==========================================
  Files          23       23              
  Lines        3083     3048      -35     
==========================================
- Hits         2501     2234     -267     
- Misses        582      814     +232     

Files Changed	Coverage Δ
src/jlgen.jl	66.55% <ø> (-3.02%)	⬇️
src/interface.jl	76.62% <100.00%> (-6.28%)	⬇️
src/ptx.jl	69.88% <100.00%> (-26.69%)	⬇️

... and 8 files with indirect coverage changes

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