bpftool: Set BPF_F_XDP_DEV_BOUND_ONLY flag non-destructively

When setting the XDP hints ifname, bpftool would set the
BPF_F_XDP_DEV_BOUND_ONLY without looking at the existing program flags,
overriding any other flag values. This was always a destructive action,
but after we change libbpf to carry the frags section flag in
prog_flags, this can impact bpftool loading of XDP frags programs.

Change the flag setting to be non-destructive by OR'ing it with the
existing flags.

Fixes: f46392ee3dec ("bpftool: Specify XDP Hints ifname when loading program")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Larysa Zaremba <larysa.zaremba@intel.com>
Link: https://lore.kernel.org/bpf/20260901-libbpf-frags-flags-v3-1-4eb6f14968b0@redhat.com