selftests/bpf: Add selftest for attaching fexit to __noreturn functions

The reuslt:

$ tools/testing/selftests/bpf/test_progs --name=fexit_noreturns
#99/1 fexit_noreturns/noreturns:OK
#99 fexit_noreturns:OK
Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Yafang Shao <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/fexit_noreturns.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include <test_progs.h>
+#include "fexit_noreturns.skel.h"
+
+void test_fexit_noreturns(void)
+{
+	RUN_TESTS(fexit_noreturns);
+}

tools/testing/selftests/bpf/progs/fexit_noreturns.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_misc.h"
+
+char _license[] SEC("license") = "GPL";
+
+SEC("fexit/do_exit")
+__failure __msg("Attaching fexit/fmod_ret to __noreturn functions is rejected.")
+int BPF_PROG(noreturns)
+{
+	return 0;
+}