Build selftests, bpftool, and libbpf in debug mode with DWARF data to
facilitate easier debugging.

In terms of impact on building and running selftests. Build is actually faster
now:

BEFORE: make -j60  380.21s user 37.87s system 1466% cpu 28.503 total
AFTER:  make -j60  345.47s user 37.37s system 1599% cpu 23.939 total

test_progs runtime seems to be the same:

BEFORE:
real    1m5.139s
user    0m1.600s
sys     0m43.977s

AFTER:
real    1m3.799s
user    0m1.721s
sys     0m42.420s

Huge difference is being able to debug issues throughout test_progs, bpftool,
and libbpf without constantly updating 3 Makefiles by hand (including GDB
seeing the source code without any extra incantations).
Signed-off-by: NAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210313210920.1959628-5-andrii@kernel.org