external-toolchain: support libraries outside of /lib

The copy_toolchain_lib_root function was making the assumption that
all libraries were stored inside the /lib directory of the sysroot
directory. However, this isn't true for certain toolchains,
particularly for the libstdc++ library.

The function is therefore reworked to find the library and its related
symlink either in /lib or /usr/lib in the sysroot, and copies it at
the same location in the target directory.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>