m68k: Don't comment out syscalls used by glibc

Define again the syscalls that are used by glibc so that it is possible to
compile a feature-complete glibc with the newest kernel headers.

Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>