tools/bpf: properly account for libbfd variations

On some platforms, in order to link against libbfd, we need to
link against liberty and even possibly libz. Account for that
in the bpftool Makefile. We now have proper feature detection
for each case, so handle each one separately.

See recent commit 14541b1e ("perf build: Don't unconditionally link the
libbfd feature test to -liberty and -lz") where I fixed feature
detection.

v2 (addressed Jakub's nits):
  * better syntax for 'else ifeq'
  * no space between ifeq args
v3:
  * use LIBS, not EXTLIBS for -DHAVE_LIBBFD_SUPPORT

Fixes: 29a9c10e

 ("bpftool: make libbfd optional")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>