[llvm] [libc] [libc][NFC] Fix "type qualifiers ignored on cast result type" GCC warning (PR #78509)

[via llvm-commits]

lntue wrote:

> > > I changed the build system to add `-frounding-math` to both CMake and Bazel. @lntue let me know if there's a better way to do this. I've checked that the flag is passed down to the command line but I haven't checked the generated code.
> > 
> > 
> > Adding that flag should work. Can you only add that flag to the updated tests in this patch for now?
> 
> Do you mean that I should add the flag only on this particular test and not globally for all `add_fp_unittest` / `libc_test`?

Yes.

https://github.com/llvm/llvm-project/pull/78509