libc/glibc: use the common start_files procedure

Use the common procedure, shared between glibc and eglibc. This requires
that glibc-specific bits be included in the shared procedure.

But still build the full libc with the glibc-specific procedure. This will
be commonalised in a future commit.

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

 reporter_name="Remy Bohmer"

 reporter_url="linux@bohmer.net"

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