[libclc] fix compiler check with --target=spirv64 and -disable-llvm-passes (#185376)
Fix "unknown target triple" errors when LLVM_TARGETS_TO_BUILD is empty.
Adding -disable-llvm-passes reduces this to a very basic sanity check
of Clang frontend. This allows the test to pass even if SPIR-V backend
is not enabled, as the frontend can still generate IR for the target.