m68k: Replace m68k-specific _[se]bss by generic __bss_{start,stop}

BSS_SECTION() provides the __bss_{start,stop} symbols, so there's no need
to wrap our own _[se]bss around it.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Greg <Ungerer&lt;gerg@uclinux.org>