[Bug debug/112849] btf: wrong BTF_KIND_DATSEC entries for extern variables without known section

["david.faust at oracle dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112849

--- Comment #1 from David Faust <david.faust at oracle dot com> ---
Simple reproducer. Note how clang does not emit an entry
in BTF_KIND_DATASEC for 'extern int a'.

$ cat a.c
extern int a;
int b;

int foo (void)
{
  return a + b;
}

$ ~/toolchains/bpf/bin/bpf-unknown-none-gcc -gbtf -c a.c -o a.o
$ /usr/sbin/bpftool btf dump file a.o
[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[2] FUNC_PROTO '(anon)' ret_type_id=1 vlen=0
[3] VAR 'a' type_id=1, linkage=extern
[4] VAR 'b' type_id=1, linkage=global
[5] FUNC 'foo' type_id=2 linkage=global
[6] DATASEC '.bss' size=0 vlen=2
        type_id=3 offset=0 size=4 (VAR 'a')
        type_id=4 offset=0 size=4 (VAR 'b')

$ ~/toolchains/llvm/bin/clang -target bpf -c -g a.c -o a.o
$ /usr/sbin/bpftool btf dump file a.o
[1] FUNC_PROTO '(anon)' ret_type_id=2 vlen=0
[2] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
[3] FUNC 'foo' type_id=1 linkage=global
[4] VAR 'a' type_id=2, linkage=extern
[5] VAR 'b' type_id=2, linkage=global
[6] DATASEC '.bss' size=0 vlen=1
        type_id=5 offset=0 size=4 (VAR 'b')