[compiler-rt] [tsan] Refine fstat{,64} interceptors (PR #86625)

[Fangrui Song via llvm-commits]

MaskRay wrote:

I've checked our internal systems and this patch seems good.

I will wait a bit to ensure things are stable and consider release/18.x backport if it's still accepting updates.

https://github.com/llvm/llvm-project/pull/86625