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>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42337 3c298f89-4303-0410-b956-a3cf2f4a3e73