[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 "Lin

[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

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