[clang] [libc] Partially implement 'rand' for the GPU (PR #66167)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Can you double check this error from the linux x86_64 bot:
> 
> ```
> /var/lib/buildkite-agent/builds/linux-56-7f758798dd-khkmx-1/llvm-project/github-pull-requests/libc/test/src/stdlib/rand_test.cpp:27:2: error: unterminated conditional directive
> --
>   | #ifndef LIBC_TARGET_ARCH_IS_GPU
>   | ^
>   | 1 error generated.
> ```

I think I broke that when I rebased the test. Will fix.

https://github.com/llvm/llvm-project/pull/66167