[BinaryFormat] Adjust OSABI functions and add unittests

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