Add fastmath flag to PTXCompilerTarget

[Zentrik]

I think this should all be fine, seems to work as intended for me.

[maleadt]

Thanks, LGTM

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -9.04% ⚠️

Comparison is base (4855eee) 82.31% compared to head (fc6c39d) 73.27%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
- Coverage   82.31%   73.27%   -9.04%     
==========================================
  Files          23       23              
  Lines        3088     3050      -38     
==========================================
- Hits         2542     2235     -307     
- Misses        546      815     +269     

Files Changed	Coverage Δ
src/ptx.jl	69.66% <33.33%> (-26.93%)	⬇️

... and 16 files with indirect coverage changes

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