Adjust #89280:
* ELFOSABI_LINUX is a historical alias that should not be used in new
code. readelf -h displays "UNIX - GNU" instead of "Linux".
* "OS" is inappropriate. Some values are architecture-specific, e.g.
ELFOSABI_ARM.
* Drop lowercase, which seems a job of the caller.
Add some unittests.
Pull Request: https://github.com/llvm/llvm-project/pull/90270
