Auto merge of rust-lang#134690 - clubby789:ci-clang-lto, r=<try>

CI: Add LTO support to clang in dist-x86_64-linux

After rust-lang/cc-rs#1279, we attempt to pass `-flto=thin` to clang. In `dist-x86_64-linux`, we don't build clang with the `LLVMgold.so` library so this fails. This attempts to resolve this
First, pass the binutils plugin include directory to Clang, [which will build the library](https://github.com/llvm/llvm-project/blob/2d6d723a85c2d007b0359c206d66cd2e5a9f00e1/llvm/docs/GoldPlugin.rst#how-to-build-it)
Second, this library depends on the *version of libstdc++ that we built* specifically. However, despite both the RPATH and LD_LIBRARY_PATH pointing to `/rustroot/lib`, we incorrectly resolve to the system libstdc++, which doesn't load.
```
# LD_DEBUG=libs,files
      2219:    file=libstdc++.so.6 [0];  needed by /rustroot/bin/../lib/LLVMgold.so [0]
      2219:    find library=libstdc++.so.6 [0]; searching
      2219:     search path=/rustroot/bin/../lib/../lib        (RPATH from file /rustroot/bin/../lib/LLVMgold.so)
      2219:      trying file=/rustroot/bin/../lib/../lib/libstdc++.so.6
      2219:     search path=/usr/lib64/tls:/usr/lib64        (system search path)
      2219:      trying file=/usr/lib64/tls/libstdc++.so.6
      2219:      trying file=/usr/lib64/libstdc++.so.6
```

Using `LD_PRELOAD` causes it to correctly load the library

I think this is probably not the most maintainable way to do this, so opening to see if this is desired and if there's a better way of doing this

Author: bors

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

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)