makefile: support libbpf symbol versioning in shared library mode

Similarly to Linux's 1bd63524593b ("libbpf: handle symbol versioning properly
for libbpf.a"), add necessary changes to build static and shared object
files separately with extra shared library flags. This allows to
properly handle symbol versioning in shared library mode, while still
having statically linkable library.

Cc: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>

src/.gitignore

@@ -2,3 +2,5 @@
 *.a
 /libbpf.pc
 /libbpf.so*
+/staticobjs
+/sharedobjs