llvm-builder: do not build clang by default (#168)

We only need LLD for cross compilation. This significantly reduces the LLVM build times in a cross compilation scenario. Update the README as a drive-by.

.github/workflows/build-revive-wasm.yml

@@ -55,13 +55,13 @@ jobs:
       - name: Build host LLVM
         if: steps.cache-llvm.outputs.cache-hit != 'true'
         run: |
-          revive-llvm build
+          revive-llvm build --llvm-projects lld --llvm-projects clang
 
       - name: Build target LLVM
         if: steps.cache-llvm.outputs.cache-hit != 'true'
         run: |
           source emsdk/emsdk_env.sh
-          revive-llvm --target-env emscripten build
+          revive-llvm --target-env emscripten build --llvm-projects lld
 
       - run: |
           rustup show