Move some setup for cg_gcc outside of the docker file

src/ci/docker/host-x86_64/x86_64-gnu-llvm-15/Dockerfile

@@ -35,11 +35,6 @@ RUN curl -sL "https://github.com/PowerShell/PowerShell/releases/download/v7.3.1/
 COPY scripts/sccache.sh /scripts/
 RUN sh /scripts/sccache.sh
 
-# Make `libgccjit.so` accessible.
-RUN ln -s /usr/lib/gcc/x86_64-linux-gnu/12/libgccjit.so /usr/lib/x86_64-linux-gnu/libgccjit.so
-# Fix rustc_codegen_gcc lto issues.
-ENV GCC_EXEC_PREFIX="/usr/lib/gcc/"
-
 # We are disabling CI LLVM since this builder is intentionally using a host
 # LLVM, rather than the typical src/llvm-project LLVM.
 ENV NO_DOWNLOAD_CI_LLVM 1
@@ -53,8 +48,7 @@ ENV RUST_CONFIGURE_ARGS \
       --build=x86_64-unknown-linux-gnu \
       --llvm-root=/usr/lib/llvm-15 \
       --enable-llvm-link-shared \
-      --set rust.thin-lto-import-instr-limit=10 \
-      --enable-new-symbol-mangling
+      --set rust.thin-lto-import-instr-limit=10
 
 COPY host-x86_64/x86_64-gnu-llvm-15/script.sh /tmp/
 

src/ci/docker/host-x86_64/x86_64-gnu-llvm-15/script.sh

@@ -2,6 +2,16 @@
 
 set -ex
 
+if [[ "${ENABLE_GCC_CODEGEN}" == "1" ]]; then
+    # Fix rustc_codegen_gcc lto issues.
+    export GCC_EXEC_PREFIX="/usr/lib/gcc/"
+    # Make `libgccjit.so` accessible to the linker.
+    ln -s /usr/lib/gcc/x86_64-linux-gnu/12/libgccjit.so /usr/lib/x86_64-linux-gnu/libgccjit.so
+    # cg_gcc doesn't support the legacy mangling so we need to enforce the new one
+    # if we run cg_gcc tests.
+    export RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --enable-new-symbol-mangling"
+fi
+
 # Only run the stage 1 tests on merges, not on PR CI jobs.
 if [[ -z "${PR_CI_JOB}" ]]; then
     # When running gcc backend tests, we need to install `libgccjit` and to not run llvm codegen