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

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

src/ci/docker/host-x86_64/dist-i686-linux/Dockerfile

@@ -50,6 +50,7 @@ COPY host-x86_64/dist-x86_64-linux/shared.sh /tmp/
 
 # Need at least GCC 5.1 to compile LLVM nowadays
 COPY host-x86_64/dist-x86_64-linux/build-gcc.sh /tmp/
+ENV GCC_VERSION=9.5.0
 RUN ./build-gcc.sh && yum remove -y gcc gcc-c++
 
 COPY scripts/cmake.sh /tmp/

src/ci/docker/host-x86_64/dist-x86_64-linux/Dockerfile

@@ -44,12 +44,14 @@ RUN mkdir -p /rustroot/bin
 ENV PATH=/rustroot/bin:$PATH
 ENV LD_LIBRARY_PATH=/rustroot/lib64:/rustroot/lib32:/rustroot/lib
 ENV PKG_CONFIG_PATH=/rustroot/lib/pkgconfig
+# Clang needs to access GCC headers to enable linker plugin LTO
 WORKDIR /tmp
 RUN mkdir /home/user
 COPY host-x86_64/dist-x86_64-linux/shared.sh /tmp/
 
 # Need at least GCC 5.1 to compile LLVM nowadays
 COPY host-x86_64/dist-x86_64-linux/build-gcc.sh /tmp/
+ENV GCC_VERSION=9.5.0
 RUN ./build-gcc.sh && yum remove -y gcc gcc-c++
 
 # LLVM 17 needs cmake 3.20 or higher.
@@ -105,3 +107,7 @@ ENV DIST_SRC 1
 ENV LIBCURL_NO_PKG_CONFIG 1
 
 ENV DIST_REQUIRE_ALL_TOOLS 1
+
+# FIXME: Without this, LLVMgold.so incorrectly resolves to the system
+# libstdc++, instead of the one we build.
+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/$GCC_VERSION/plugin/include/" \
       -DC_INCLUDE_DIRS="$INC"
 
 hide_output make -j$(nproc)

src/ci/docker/host-x86_64/dist-x86_64-linux/build-gcc.sh

@@ -1,10 +1,11 @@
 #!/usr/bin/env bash
-set -ex
+set -eux
 
 source shared.sh
 
 # Note: in the future when bumping to version 10.1.0, also take care of the sed block below.
-GCC=9.5.0
+# This version is specified in the Dockerfile
+GCC=$GCC_VERSION
 
 curl https://ftp.gnu.org/gnu/gcc/gcc-$GCC/gcc-$GCC.tar.xz | xzcat | tar xf -
 cd gcc-$GCC