enable btf for offload.o & test.o

The objdump -x visible changes between old and new versions of the
mainline shipped .o files are really very minimal: just the inclusion
of a new .BTF section and changes/removals of some 'l' entries from
the symbol table.  However, it turns out a change to symbol ordering
is incompatible with BpfLoader <v0.10 which doesn't know to skip
non-function symbols, and as such enabling btf requires a little
bit of gymnastics.

After:
  $ adbz shell ls -l /apex/com.android.tethering/etc/bpf/*.o
  -rw-r--r-- 1 system system 118352 1969-12-31 16:00 /apex/com.android.tethering/etc/bpf/offload.o
  -rw-r--r-- 1 system system 123424 1969-12-31 16:00 /apex/com.android.tethering/etc/bpf/offload@btf.o
  -rw-r--r-- 1 system system   2232 1969-12-31 16:00 /apex/com.android.tethering/etc/bpf/test.o
  -rw-r--r-- 1 system system   6376 1969-12-31 16:00 /apex/com.android.tethering/etc/bpf/test@btf.o

$ adbz shell logcat -d | egrep offload.*[.]o
07-15 13:10:43.358     0     0 D LibBpfLoader: Loading critical for tethering ELF object /apex/com.android.tethering/etc/bpf/offload.o with license Apache 2.0
07-15 13:10:43.359     0     0 I LibBpfLoader: BpfLoader version 0x00019 ignoring ELF object /apex/com.android.tethering/etc/bpf/offload.o with max ver 0x00019
07-15 13:10:43.359     0     0 I bpfloader: Loaded object: /apex/com.android.tethering/etc/bpf/offload.o
07-15 13:10:43.374     0     0 D LibBpfLoader: Loading critical for tethering ELF object /apex/com.android.tethering/etc/bpf/offload@btf.o with license Apache 2.0
07-15 13:10:43.375     0     0 I LibBpfLoader: BpfLoader version 0x00019 processing ELF object /apex/com.android.tethering/etc/bpf/offload@btf.o with ver [0x00019,0x10000)
07-15 13:10:43.452     0     0 D LibBpfLoader: map_fd found at 0 is 6 in /apex/com.android.tethering/etc/bpf/offload@btf.o
...

Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Id658818d1d42763358747523615b7918d312588e