libbpf: make libbpf_errno.c into more generic libbpf_utils.c

Libbpf is missing one convenient place to put common "utils"-like code
that is generic and usable from multiple places. Use libbpf_errno.c as
the base for more generic libbpf_utils.c.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20251001171326.3883055-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>

Author: anakryiko

tools/lib/bpf/Build

@@ -1,4 +1,4 @@
-libbpf-y := libbpf.o bpf.o nlattr.o btf.o libbpf_errno.o str_error.o \
+libbpf-y := libbpf.o bpf.o nlattr.o btf.o libbpf_utils.o str_error.o \
 	    netlink.o bpf_prog_linfo.o libbpf_probes.o hashmap.o \
 	    btf_dump.o ringbuf.o strset.o linker.o gen_loader.o relo_core.o \
 	    usdt.o zip.o elf.o features.o btf_iter.o btf_relocate.o