Set some environment variables value only if ENABLE_GCC_CODEGEN is set

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

@@ -35,10 +35,8 @@ 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.
+# Make `libgccjit.so` accessible to the linker.
 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.
@@ -48,13 +46,21 @@ ENV NO_DOWNLOAD_CI_LLVM 1
 # be missing.
 ENV IS_NOT_LATEST_LLVM 1
 
+# If `ENABLE_GCC_CODEGEN` is set and not empty, we add the `--enable-new-symbol-mangling`
+# argument to `RUST_CONFIGURE_ARGS` and set the `GCC_EXEC_PREFIX` environment variable.
+# `cg_gcc` doesn't support the legacy mangling so we need to enforce the new one
+# if we run `cg_gcc` tests.
+ENV USE_NEW_MANGLING ${ENABLE_GCC_CODEGEN:+ --enable-new-symbol-mangling}
+# Fix rustc_codegen_gcc lto issues.
+ENV GCC_EXEC_PREFIX ${ENABLE_GCC_CODEGEN:+/usr/lib/gcc/}
+
 # Using llvm-link-shared due to libffi issues -- see #34486
 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
+      $USE_NEW_MANGLING \
+      --set rust.thin-lto-import-instr-limit=10
 
 COPY host-x86_64/x86_64-gnu-llvm-15/script.sh /tmp/