[llvm] [X86] Declare command line options in TableGen (PR #228327)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> Migration PRs should faithfully translate existing options so they don't break downstream users. Prefix x86-specific library options with `x86-` (some don't), and handle the removal of obsolete options in separate PRs.

Yes, happy to do both, what kind of time limit are you running to? Either we just dump everything across now or I try to do some/all the cleanup first.

https://github.com/llvm/llvm-project/pull/228327