Add //go:build go1.23 constraint to skip compilation on Go 1.21/1.22
where runtime.(*Func).Name is not implemented in llgo, causing linker
errors when internal/bisect is pulled in as a dependency.
The demo works correctly on Go 1.23+ where the dependency chain or
internal/bisect behavior avoids calling the unimplemented method.
Fixes compatibility issue reported in PR review.
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- Moved full overlay file from runtime/_overlay/go/build/build.go to
runtime/internal/go/build/build.go to make it clearer what's being overridden
- Updated runtime/overlay.go to embed from new location
- Created demo under _demo/go/gobuild to demonstrate go/build package working with llgo
- The overlay still contains the full file but is now organized under runtime/internal
following the project's pattern for patched standard library packages
Fixes#1346
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