add libelf from elftoolchain r3669

A 2 clause BSD licensed implementation of the SVR4 ELF API also
implemented by mr511.de libelf (devel/libelf in ports) and elfutils.

It is being added to base to allow Mesa to use it in future.

shlib major is higher than devel/libelf and pkg-config version is 0.8.2
to pass a glib2 configure test so this can replace the use of
devel/libelf in ports.