[llvm] adds conversion functions for EI_OSABI in elf (PR #89280)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> I feel like some basic unit testing for this should really exist. I'll acknowledge that there's probably not a huge amount of point testing every single OS/OSABI pair, but you could test a few arbitrary cases, plus the "none" and default cases.

Agreed. Created #90270 for unittests and adjusting ELFOSABI_LINUX (historical alias, deprecated https://sourceware.org/bugzilla/show_bug.cgi?id=12913)

https://github.com/llvm/llvm-project/pull/89280