[llvm] [llvm][docs] Update list of working Linux host platforms (PR #149503)

[David Spickett via llvm-commits]

DavidSpickett wrote:

@llvm/issue-subscribers-backend-loongarch , anyone able to confirm my assertion here?

@tedwoodward Hexagon is a target of llvm, and I see some `hexagon-linux` triples in the sources but do you actually host llvm on Hexagon itself? I wasn't sure but if yes, I can add it too.

https://github.com/llvm/llvm-project/pull/149503