tests: Fix compiler-tests.sh for no build args passed case

When a user ran "tests/compiler-tests.sh" without passing any arguments,
the compiler tests would fail with:

```
scons: Reading SConscript files ...
Error: No existing build directory and no variant for /gem5
```

However, when passed with arguments, such as:

```
./tests/compiler-tests.sh -j6
```

the tests passed.

The fix for this is to merge the "$build_out" and "$build_args" into a
single string when executing the docker. I do not know exactly why this
works, but it does fix the error.

Change-Id: Ibcd316668b60fb7706f0ee05ab6dadf56228319d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/66631
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/compiler-tests.sh

@@ -135,7 +135,7 @@ 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>"${build_stderr}"
             result=$?