buildtools-tarball: Add an ld.so.conf for nativesdk-binutils

We need to search our own libdirs, then fall back to the system ones as our
customised dynamic loader will. Have ld.so.conf reflect that.

This ensures that binutils finds libraries here when linking too.

(From OE-Core rev: ab729c36)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>