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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Not sure if this is the correct place to make these modifications. The comment at the end seems like it might be, but there could definitely be a better way to set this up as I'm not that familiar with Bazel. I didn't find much in the docs there about selecting specific defines based on compiler versions/other platform specific details, but it could definitely be there.

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