[compiler-rt] [NFC][sanitizer] Move ThreadDescriptorSize into GLIBC/FREEBSD block (PR #108913)

[Rainer Orth via llvm-commits]

rorth wrote:

This patch broke the [Solaris/sparcv9](https://lab.llvm.org/buildbot/#/builders/13/builds/2323) and [Solaris/amd64](https://lab.llvm.org/staging/#/builders/120/builds/1883) buildbots again.  Doesn't seem NFC to me ;-(

https://github.com/llvm/llvm-project/pull/108913