[compiler-rt] On Windows, silence warning when building with latest MSVC

This fixes:
```
[3334/7449] Building C object projects\compiler-rt\lib\builtins\CMakeFiles\clang_rt.builtins-x86_64.dir\extendsfdf2.c.obj
C:\git\llvm-project\compiler-rt\lib\builtins\fp_extend_impl.inc(63): warning C4018: '<': signed/unsigned mismatch
```