Bug 1758780 - Avoid building LLVM when building wasi compiler-rt. r=firefox-build-system-reviewers,mhentges

Instead, do the same trick as for the wasi sysroot, and reuse clang
itself.

Differential Revision: https://phabricator.services.mozilla.com/D143171

taskcluster/ci/toolchain/compiler-rt.yml

@@ -223,3 +223,5 @@ wasm32-wasi-compiler-rt-13:
         fetch:
             - clang-13
             - wasi-sdk
+        toolchain:
+            - linux64-clang-13-stage1

taskcluster/scripts/misc/build-compiler-rt-wasi.sh

@@ -7,6 +7,17 @@ dir=${artifact%.tar.*}
 cd $MOZ_FETCHES_DIR/wasi-sdk
 LLVM_PROJ_DIR=$MOZ_FETCHES_DIR/llvm-project
 
+mkdir -p build/install/wasi
+# The wasi-sdk build system wants to build clang itself. We trick it into
+# thinking it did, and put our own clang where it would have built its own.
+ln -s $MOZ_FETCHES_DIR/clang build/llvm
+touch build/llvm.BUILT
+
+# The wasi-sdk build system wants a clang and an ar binary in
+# build/install/$PREFIX/bin
+ln -s $MOZ_FETCHES_DIR/clang/bin build/install/wasi/bin
+ln -s llvm-ar build/install/wasi/bin/ar
+
 # Build compiler-rt
 make \
   LLVM_PROJ_DIR=$LLVM_PROJ_DIR \