libbpf: Add support for global percpu data

Add support for global percpu data in libbpf by adding a new ".percpu"
section, similar to ".data". It enables efficient handling of percpu
global variables in bpf programs.

When generating loader for lightweight skeleton, update the percpu_array
map used for global percpu data using BPF_F_ALL_CPUS, in order to update
values across all CPUs using one value slot.

Unlike global data, the mmaped data for global percpu data will be marked
as read-only after populating the percpu_array map. Thereafter, users can
read those initialized percpu data after loading prog. If they want to
update the percpu data after loading prog, they have to update the
percpu_array map using key=0 instead.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260813152324.97937-6-leon.hwang@linux.dev

src/bpf_gen_internal.h

@@ -65,7 +65,8 @@ void bpf_gen__prog_load(struct bpf_gen *gen,
 			enum bpf_prog_type prog_type, const char *prog_name,
 			const char *license, struct bpf_insn *insns, size_t insn_cnt,
 			struct bpf_prog_load_opts *load_attr, int prog_idx);
-void bpf_gen__map_update_elem(struct bpf_gen *gen, int map_idx, void *value, __u32 value_size);
+void bpf_gen__map_update_elem(struct bpf_gen *gen, int map_idx, void *value, __u32 value_size,
+			      __u64 flags);
 void bpf_gen__map_freeze(struct bpf_gen *gen, int map_idx);
 void bpf_gen__record_attach_target(struct bpf_gen *gen, const char *name, enum bpf_attach_type type);
 void bpf_gen__record_extern(struct bpf_gen *gen, const char *name, bool is_weak,