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

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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?

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