Add LTO support to clang in CI

Author: clubby789

Diff for: src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile

@@ -104,3 +104,5 @@ ENV DIST_SRC 1
 ENV LIBCURL_NO_PKG_CONFIG 1
 
 ENV DIST_REQUIRE_ALL_TOOLS 1
+
+ENV LD_PRELOAD=/rustroot/lib64/libstdc++.so.6

Diff for: src/ci/docker/host-x86_64/dist-x86_64-linux/build-clang.sh

@@ -39,6 +39,7 @@ hide_output \
       -DLLVM_INCLUDE_TESTS=OFF \
       -DLLVM_INCLUDE_EXAMPLES=OFF \
       -DLLVM_ENABLE_PROJECTS="clang;lld;compiler-rt;bolt" \
+      -DLLVM_BINUTILS_INCDIR="/rustroot/lib/gcc/x86_64-pc-linux-gnu/9.5.0/plugin/include/" \
       -DC_INCLUDE_DIRS="$INC"
 
 hide_output make -j$(nproc)