binutils: link libbfd and libopcodes dynamically again

Build libbfd and libopcodes as shared libraries, else they get linked
statically into every binary, increasing each binary size by ~650kB.

Reduces binutils package size to old expected values.

Fixes #17060.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 42337