libbpf: fix sym_is_subprog() logic for weak global subprogs

[ Upstream commit 4073213488be542f563eb4b2457ab4cbcfc2b738 ]

sym_is_subprog() is incorrectly rejecting relocations against *weak*
global subprogs. Fix that by realizing that STB_WEAK is also a global
function.

While it seems like verifier doesn't support taking an address of
non-static subprog right now, it's still best to fix support for it on
libbpf side, otherwise users will get a very confusing error during BPF
skeleton generation or static linking due to misinterpreted relocation:

  libbpf: prog 'handle_tp': bad map relo against 'foo' in section '.text'
  Error: failed to open BPF object file: Relocation failed

It's clearly not a map relocation, but is treated and reported as such
without this fix.

Fixes: 53eddb5e04 ("libbpf: Support subprog address relocation")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20241009011554.880168-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

tools/lib/bpf/libbpf.c

@@ -3586,7 +3586,7 @@ static bool sym_is_subprog(const Elf64_Sym *sym, int text_shndx)
 		return true;
 
 	/* global function */
-	return bind == STB_GLOBAL && type == STT_FUNC;
+	return (bind == STB_GLOBAL || bind == STB_WEAK) && type == STT_FUNC;
 }
 
 static int find_extern_btf_id(const struct btf *btf, const char *ext_name)