selftests: mm: add s390 to ARCH check

commit 0518dbe9 ("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: 0518dbe9 ("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>

tools/testing/selftests/mm/Makefile

@@ -110,7 +110,7 @@ endif
 
 endif
 
-ifneq (,$(filter $(ARCH),arm64 ia64 mips64 parisc64 powerpc riscv64 s390x sparc64 x86_64))
+ifneq (,$(filter $(ARCH),arm64 ia64 mips64 parisc64 powerpc riscv64 s390x sparc64 x86_64 s390))
 TEST_GEN_FILES += va_high_addr_switch
 TEST_GEN_FILES += virtual_address_range
 TEST_GEN_FILES += write_to_hugetlbfs