selftests/bpf: Make selftest compilation work on clang 11

We can't compile test_core_reloc_module.c selftest with clang 11, compile
fails with:

  CLNG-LLC [test_maps] test_core_reloc_module.o
  progs/test_core_reloc_module.c:57:21: error: use of unknown builtin \
  '__builtin_preserve_type_info' [-Wimplicit-function-declaration]
   out->read_ctx_sz = bpf_core_type_size(struct bpf_testmod_test_read_ctx);

Skipping these tests if __builtin_preserve_type_info() is not supported
by compiler.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201209142912.99145-1-jolsa@kernel.org

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

@@ -40,6 +40,7 @@ int BPF_PROG(test_core_module_probed,
 	     struct task_struct *task,
 	     struct bpf_testmod_test_read_ctx *read_ctx)
 {
+#if __has_builtin(__builtin_preserve_enum_value)
 	struct core_reloc_module_output *out = (void *)&data.out;
 	__u64 pid_tgid = bpf_get_current_pid_tgid();
 	__u32 real_tgid = (__u32)(pid_tgid >> 32);
@@ -61,6 +62,9 @@ int BPF_PROG(test_core_module_probed,
 	out->len_exists = bpf_core_field_exists(read_ctx->len);
 
 	out->comm_len = BPF_CORE_READ_STR_INTO(&out->comm, task, comm);
+#else
+	data.skip = true;
+#endif
 
 	return 0;
 }
@@ -70,6 +74,7 @@ int BPF_PROG(test_core_module_direct,
 	     struct task_struct *task,
 	     struct bpf_testmod_test_read_ctx *read_ctx)
 {
+#if __has_builtin(__builtin_preserve_enum_value)
 	struct core_reloc_module_output *out = (void *)&data.out;
 	__u64 pid_tgid = bpf_get_current_pid_tgid();
 	__u32 real_tgid = (__u32)(pid_tgid >> 32);
@@ -91,6 +96,9 @@ int BPF_PROG(test_core_module_direct,
 	out->len_exists = bpf_core_field_exists(read_ctx->len);
 
 	out->comm_len = BPF_CORE_READ_STR_INTO(&out->comm, task, comm);
+#else
+	data.skip = true;
+#endif
 
 	return 0;
 }