Wire up the clone3() syscall for m68k. The special entry point is done in
assembler as was done for clone() as well. This is needed because all
registers need to be saved. The C wrapper then calls the generic
sys_clone3() with the correct arguments.

Tested on A1200 using the simple test program from:

  https://lore.kernel.org/lkml/20190716130631.tohj4ub54md25dys@brauner.io/Signed-off-by: Kars de Jong <jongk@linux-m68k.org>
Link: https://lore.kernel.org/r/20191124195225.31230-1-jongk@linux-m68k.orgSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>