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

[Reid Kleckner via llvm-commits]

rnk wrote:

I guess in general this does require a configure check, since GCC __has_builtin returns true for this, but then errors out during code generation:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96952#c2

We should have to replicate knowledge of which targets GCC supports this builtin on in order to forego the configure check.

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