[compiler-rt] [compiler-rt] Fix declarations of builtins in test files (PR #161222)

[Akira Hatanaka via llvm-commits]

ahatanak wrote:

We need this change if we want to run the tests on `x86_64` (see https://github.com/llvm/llvm-project/pull/160687#issuecomment-3339790472).

https://github.com/llvm/llvm-project/pull/161222