[clang] ohos: fix ohos.c test case error with LLVM_ENABLE_PER_TARGET_RUNTIME_… (PR #121484)

[Carlo Cabrera via cfe-commits]

carlocab wrote:

> Note: OHOS driver doesn't support the old layout, compiler-rt for ${arch}-linux-unknown-ohos targets have to be built with LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON

Can we make `cmake` error out if you try to build for `*-linux-unknown-ohos` with `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF`?

https://github.com/llvm/llvm-project/pull/121484