Move hardcoded build tags and ldflags from Makefile, Dockerfile, CI
workflows, and local build scripts into canonical files under release/:
- release/DEFAULT_BUILD_TAGS (Linux common archs, Darwin, Android)
- release/DEFAULT_BUILD_TAGS_WINDOWS (includes with_purego)
- release/DEFAULT_BUILD_TAGS_OTHERS (no with_naive_outbound)
- release/LDFLAGS (shared linker flags)
