The .go and .s assembly files are auto-generated upstream and shipped as-is.

Regeneration is not feasible during Debian build because:

 - gocc (github.com/kelindar/gocc) is required but not packaged in Debian
 - gocc invokes clang with cross-architecture auto-vectorization targets
 - Even upstream does not routinely regenerate: the gocc commands in
   codegen/generate.sh are commented out

Evidence in the source tree:

 - codegen/README.md: documents gocc dependency
 - codegen/generate.sh: gocc invocations commented out
 - simd_avx2_amd64.go header: "AUTO-GENERATED BY GOCC -- DO NOT EDIT"

Reported upstream here:

https://github.com/kelindar/simd/issues/7
