Clean up multi-piped sed command in Makefile

[oskooi]

The multi-piped sed on line 37 of src/Makefile.am is replaced with a single sed invocation using multiple -e flags. The new form is easier to read, extend (just add another -e line), and run as a single process instead of five.

The regenerated step_generic_stride1.cpp is byte-identical to the original, confirming correctness.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.87%. Comparing base (f29a8c7) to head (cb81016).
⚠️ Report is 120 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3187      +/-   ##
==========================================
+ Coverage   73.81%   73.87%   +0.05%     
==========================================
  Files          18       18              
  Lines        5423     5458      +35     
==========================================
+ Hits         4003     4032      +29     
- Misses       1420     1426       +6     

see 2 files with indirect coverage changes

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