[BPF] emit BTF sections only if debuginfo available

Currently, without -g, BTF sections may still be emitted with
data sections, e.g., for linux kernel bpf selftest
test_tcp_check_syncookie_kern.c issue discovered by Martin
as shown below.

-bash-4.4$ bpftool btf dump file test_tcp_check_syncookie_kern.o
[1] VAR 'results' type_id=0, linkage=global-alloc
[2] VAR '_license' type_id=0, linkage=global-alloc
[3] DATASEC 'license' size=0 vlen=1
        type_id=2 offset=0 size=4
[4] DATASEC 'maps' size=0 vlen=1
        type_id=1 offset=0 size=28

Let disable BTF generation if no debuginfo, which is
the original design.

Signed-off-by: Yonghong Song <yhs@fb.com>

Differential Revision: https://reviews.llvm.org/D61826

llvm-svn: 360556