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

[Nick Desaulniers via llvm-commits]

nickdesaulniers wrote:

> > 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.

+1, then you can remove all uses of `volatile` in that test as well.

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