[llvm-branch-commits] [clang][x86] Support -mtls-dialect for x86_64 targets (PR #84086)

[Paul Kirth via llvm-branch-commits]

https://github.com/ilovepi created https://github.com/llvm/llvm-project/pull/84086

This patch allows -mtls-dialect= to take `desc`, `trad`, `gnu` and
`gnu2` options. `desc` and `gnu2` can be used for TLSDESC code
generation, while `trad` and `gnu` maintain the existing TLS behavior.