libc: get rid of libc_finish

At long last, we no longer have any libc that requries a libc_finish.
Yeah!

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 reporter_name="Remy Bohmer"

 reporter_url="linux@bohmer.net"

 reporter_comment="Example for building a toolchain for m68000 bare metal targets"