Ability to run the test suite with a clang that only includes the wasm backend #28043

Currently the build of clang we ship in EMSDK includes the native backend in addition to Wasm.
For EMSDK, it would be nice to ship a build of clang that includes only the Wasm backend. No other backend is needed for regular emscripten use, so we should be able to do that, and have a smaller package.
However there a few tests that do use a native (or strictly x86) clang to compare execution against, or for benchmarking against native. Running with EMTEST_LACKS_NATIVE_CLANG skips them (we use this e.g. on Windows Chromium CI runs).
To keep these working, we can use a clang that's separate from the one that builds Wasm. Perhaps EMTEST_NATIVE_CLANG=/path/to/clang, and then use /usr/bin/clang or the Chromium host clang as needed. It could maybe even replace EMTEST_LACKS_NATIVE_CLANG.