[compiler-rt] [sanitizer_common] Provide dummy ThreadDescriptorSize on Solaris (PR #109285)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

Oh, I was confused how Solaris references to the function are possible, I looked through the code before landing 2c69a09.

But the reason is that some references to the function are guarded with `if (SANITIZER_GLIBC)` instead of `#ifdef SANITIZER_GLIBC`.

https://github.com/llvm/llvm-project/pull/109285