build-compiler-rt: Avoid failing for --build-sanitizer if only building for e.g. aarch64

If nothing was built and installed, the installation merging command
fails with an error like this:

    cp: cannot stat '/tmp/tmp.OAj9E7UBhI/install/.': No such file or directory

Therefore, skip this step if nothing really was built and installed.

build-compiler-rt.sh

@@ -130,9 +130,16 @@ for arch in $ARCHS; do
     if [ -n "$SANITIZERS" ]; then
         mv "${WORKDIR}/install/lib/windows/"*.dll "$PREFIX/$arch-w64-mingw32/bin"
     fi
+    INSTALLED=1
     cd ..
 done
 
+if [ -z "$INSTALLED" ]; then
+    # Don't try to move the installed files in place, if nothing was
+    # installed (e.g. if building with --build-sanitizers but not for x86).
+    exit 0
+fi
+
 if [ -h "$CLANG_RESOURCE_DIR/include" ]; then
     # symlink to system headers - skip copy
     rm -rf ${WORKDIR}/install/include