lapack: add Dtgsy2/Dtgsyl and tests

[soypat]

Part of #1651

This PR candidate replaces #1886 and #1741.

Notice lapack package LocalLookAhead MaximizeNormXJob has been changed from 0 to 1 to better match Dlatdf's caller convention Dtgsy2.

[codecov-commenter]

Codecov Report

Patch coverage: 75.76% and project coverage change: +0.11% 🎉

Comparison is base (93223b4) 74.00% compared to head (d98e9cf) 74.11%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1891      +/-   ##
==========================================
+ Coverage   74.00%   74.11%   +0.11%     
==========================================
  Files         526      528       +2     
  Lines       63210    63828     +618     
==========================================
+ Hits        46778    47307     +529     
- Misses      13854    13915      +61     
- Partials     2578     2606      +28     

Files Changed	Coverage Δ
lapack/gonum/dtgsyl.go	57.14% <57.14%> (ø)
lapack/gonum/dtgsy2.go	83.48% <83.48%> (ø)
lapack/gonum/ilaenv.go	43.40% <100.00%> (+1.38%)	⬆️

... and 1 file with indirect coverage changes

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