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

[Rainer Orth via llvm-commits]

rorth 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.

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