bpf: Fix selftest compilation on clang 11

Before this patch, profiler.inc.h wouldn't compile with clang-11 (before
the __builtin_preserve_enum_value LLVM builtin was introduced in
https://reviews.llvm.org/D83242).

Another test that uses this builtin (test_core_enumval) is conditionally
skipped if the compiler is too old. In that spirit, this patch inhibits
part of populate_cgroup_info(), which needs this CO-RE builtin. The
selftests build again on clang-11.

The affected test (the profiler test) doesn't pass on clang-11 because
it's missing https://reviews.llvm.org/D85570, but at least the test suite
as a whole compiles. The test's expected failure is already called out in
the README.
Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Florian Lehner <dev@der-flo.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20201125035255.17970-1-andreimatei1@gmail.com

tools/testing/selftests/bpf/progs/profiler.inc.h

@@ -256,6 +256,7 @@ static INLINE void* populate_cgroup_info(struct cgroup_data_t* cgroup_data,
 		BPF_CORE_READ(task, nsproxy, cgroup_ns, root_cset, dfl_cgrp, kn);
 	struct kernfs_node* proc_kernfs = BPF_CORE_READ(task, cgroups, dfl_cgrp, kn);
 
+#if __has_builtin(__builtin_preserve_enum_value)
 	if (ENABLE_CGROUP_V1_RESOLVER && CONFIG_CGROUP_PIDS) {
 		int cgrp_id = bpf_core_enum_value(enum cgroup_subsys_id___local,
 						  pids_cgrp_id___local);
@@ -275,6 +276,7 @@ static INLINE void* populate_cgroup_info(struct cgroup_data_t* cgroup_data,
 			}
 		}
 	}
+#endif
 
 	cgroup_data->cgroup_root_inode = get_inode_from_kernfs(root_kernfs);
 	cgroup_data->cgroup_proc_inode = get_inode_from_kernfs(proc_kernfs);