opcodes: link against libbfd.la for rpath deps

The reason opcodes doesn't link against libbfd.la is to workaround a
libtool bug where it uses installed -L paths ahead of DESTDIR paths.
The downside is that the library itself lacks rpath tags to find the
right version of libbfd.so.

Since Gentoo has patched the libtool bug for a while, we don't need
the workaround.  Use the .la file so we get the rpath tags.

URL: https://bugs.gentoo.org/563934