[compiler-rt] [sanitizer] Extract SANITIZER_FREEBSD version of ThreadDescriptorSizeFallback (PR #109743)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

> FWIW, this does indeed fix the build failure on FreeBSD 14.0, without affecting Solaris. Unfortunately, the build fails later for different reasons:
> 
> ```
> FAILED: projects/compiler-rt/lib/builtins/CMakeFiles/clang_rt.builtins-i386.dir/extendhfsf2.c.o
> [...]
> In file included from compiler-rt/lib/builtins/extendhfsf2.c:11:
> In file included from compiler-rt/lib/builtins/fp_extend_impl.inc:38:
> compiler-rt/lib/builtins/fp_extend.h:57:9: error
> : _Float16 is not supported on this target
>    57 | typedef _Float16 src_t;
>       |         ^
> 1 error generated.
> ```
> 
> While `_Float16` is supported for 64-bit x86, it isn't for 32-bit (since that doesn't support SSE2 for whatever reason). However, the `builtins` `CMakeFile.list` only checks for `_Float16` support for the default multilib, missing this issue.

_Float16 is unrelated to the PR issue?

https://github.com/llvm/llvm-project/pull/109743