selftests/bpf: Enable tests for atomics with cpuv4

When looking at Alexei's patch ([1]) which added tests for atomics,
I noticed that the tests will be skipped with cpuv4. For example,
with latest llvm19, I see:
  [root@arch-fb-vm1 bpf]# ./test_progs -t arena_atomics
  #3/1     arena_atomics/add:OK
  ...
  #3/7     arena_atomics/xchg:OK
  #3       arena_atomics:OK
  Summary: 1/7 PASSED, 0 SKIPPED, 0 FAILED
  [root@arch-fb-vm1 bpf]# ./test_progs-cpuv4 -t arena_atomics
  #3       arena_atomics:SKIP
  Summary: 1/0 PASSED, 1 SKIPPED, 0 FAILED
  [root@arch-fb-vm1 bpf]#

It is perfectly fine to enable atomics-related tests for cpuv4.
With this patch, I have
  [root@arch-fb-vm1 bpf]# ./test_progs-cpuv4 -t arena_atomics
  #3/1     arena_atomics/add:OK
  ...
  #3/7     arena_atomics/xchg:OK
  #3       arena_atomics:OK
  Summary: 1/7 PASSED, 0 SKIPPED, 0 FAILED

  [1] https://lore.kernel.org/r/20240405231134.17274-2-alexei.starovoitov@gmail.comSigned-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240410153326.1851055-1-yonghong.song@linux.devSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/Makefile

@@ -658,7 +658,7 @@ $(eval $(call DEFINE_TEST_RUNNER,test_progs,no_alu32))
 # Define test_progs-cpuv4 test runner.
 ifneq ($(CLANG_CPUV4),)
 TRUNNER_BPF_BUILD_RULE := CLANG_CPUV4_BPF_BUILD_RULE
-TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS)
+TRUNNER_BPF_CFLAGS := $(BPF_CFLAGS) $(CLANG_CFLAGS) -DENABLE_ATOMICS_TESTS
 $(eval $(call DEFINE_TEST_RUNNER,test_progs,cpuv4))
 endif