tests: Fix compiler-tests.sh build args passing

Reverts this fix:
https://gem5-review.googlesource.com/c/public/gem5/+/66631

While this did fix the case where no build args were passed, it broke
the case where build args were passed.

This fix ensures the script works in both cases.

Change-Id: I6cc8cc0c2a10c801d4a59e54b070383ac8ee93ae
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/66772
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/compiler-tests.sh

@@ -76,7 +76,13 @@ builds_per_compiler=1
 base_url="gcr.io/gem5-test"
 
 # Arguments passed into scons on every build target test.
-build_args="$@"
+if [ $# -eq 0 ];then
+    # If none is sepcified by the user we pass "-j1" (compile on one thread).
+    # If `build_args` is left as an empty string, this script will fail.
+    build_args="-j1"
+else
+    build_args="$@"
+fi
 
 # Testing directory variables
 mkdir -p "${build_dir}" # Create the build directory if it doesn't exist.
@@ -135,7 +141,7 @@ for compiler in ${images[@]}; do
                 docker run --rm -v "${gem5_root}":"/gem5" -u $UID:$GID \
                     -w /gem5 --memory="${docker_mem_limit}" $repo_name \
                     /usr/bin/env python3 /usr/bin/scons --ignore-style \
-                    "${build_out} ${build_args}"
+                    "${build_out}" "${build_args}"
             }>"${build_stdout}" 2>"${build_stderr}"
             result=$?