libbpf, selftests/bpf: Adjust libbpf, bpftool, selftests to match LLVM

The selftests use
to tell LLVM about special pointers. For LLVM there is nothing "arena"
about them. They are simply pointers in a different address space.
Hence LLVM diff llvm/llvm-project#85161 renamed:
. macro __BPF_FEATURE_ARENA_CAST -> __BPF_FEATURE_ADDR_SPACE_CAST
. global variables in __attribute__((address_space(N))) are now
  placed in section named ".addr_space.N" instead of ".arena.N".

Adjust libbpf, bpftool, and selftests to match LLVM.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20240315021834.62988-3-alexei.starovoitov@gmail.com

src/gen.c

@@ -121,7 +121,7 @@ static bool get_datasec_ident(const char *sec_name, char *buf, size_t buf_sz)
 	int i, n;
 
 	/* recognize hard coded LLVM section name */
-	if (strcmp(sec_name, ".arena.1") == 0) {
+	if (strcmp(sec_name, ".addr_space.1") == 0) {
 		/* this is the name to use in skeleton */
 		snprintf(buf, buf_sz, "arena");
 		return true;