binutils: enable plugin support

binutils: enable plugin support

--enable-plugin is necessary for gcc-ar, gcc-nm and gcc-ranlib to work, which
must be used with GCC 4.9 for LTO to work.

Without this option, gcc-ar etc. will just fail with

    sorry - this program has been built without plugin support

Using the normal ar from binutils with GCC 4.9 and -flto will cause linking
with static "convenience" libraries to fail.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43783 3c298f89-4303-0410-b956-a3cf2f4a3e73