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

[Reid Kleckner via llvm-commits]

rnk wrote:

> Are you suggesting to do this through Bazel or still suggesting to sink it into the BenchmarkRunner.cpp file? I'd like to avoid changes there if possible as I believe the CMake check is a better solution than replicating that information with preprocessor macros.

I tend to take the opposite stance, configure-time checks are expensive and we pay for them on almost every build. IMO we should have a higher bar for adding new checks and we should ask ourselves what checks we can delete and replace with pre-processor conditionals. But, that's just my opinion, and I don't want to burden folks with work.

> If that's the case, then I can just add it to the Linux x86-64 defines in `./utils/bazel/llvm-project-overlay/llvm/config.bzl`. I guess gcc 10 is >3 years old at this point.

I like that solution.

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