tools/resolve_btfids: Build libbpf and libsubcmd in separate directories

Setting up separate build directories for libbpf and libpsubcmd,
so it's separated from other objects and we don't get them mixed
in the future.

It also simplifies cleaning, which is now simple rm -rf.

Also there's no need for FEATURE-DUMP.libbpf and bpf_helper_defs.h
files in .gitignore anymore.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210205124020.683286-2-jolsa@kernel.org