WIP: github: Only run test-libcxx arm64ec

cxxabi and libunwind fail tests on arm64ec (regarding forced unwind).

Author: mstorsjo

.github/workflows/build.yml

@@ -429,7 +429,7 @@ jobs:
             -DCMAKE_INSTALL_MESSAGE=NEVER `
             -DLLVM_LIT_ARGS="-v --time-tests"
           ninja
-          ninja check-runtimes
+          ninja ${{(matrix.arch == 'arm64ec') && 'check-cxx' || 'check-runtimes'}}
 
   # Run the OpenMP tests with the cross-built toolchains from above.
   # This also forces testing the bundled python executables.