[llvm] [Bazel] Define BUILTIN_THREAD_POINTER where appropriate on x86-64 linux (PR #74574)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> I like that solution.

Thanks for working with me to find a solution. I've implemented that and added a comment explaining the details. Please let me know if there are any refactorings/style issues with how I've done it as I'm not exactly sure what the canonical Bazel way to do this would be.

https://github.com/llvm/llvm-project/pull/74574