selftests: mm: add s390 to ARCH check

selftests: mm: add s390 to ARCH check

commit 0518dbe97fe6 ("selftests/mm: fix cross compilation with LLVM")
changed the env variable for the architecture from MACHINE to ARCH.

This is preventing 3 required TEST_GEN_FILES from being included when
cross compiling s390x and errors when trying to run the test suite.  This
is due to the ARCH variable already being set and the arch folder name
being s390.

Add "s390" to the filtered list to cover this case and have the 3 files
included in the build.

Link: https://lkml.kernel.org/r/20240724213517.23918-1-npache@redhat.com
Fixes: 0518dbe97fe6 ("selftests/mm: fix cross compilation with LLVM")
Signed-off-by: Nico Pache <npache@redhat.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>