m68k: use generic code for ptrace requests

Remove all but PTRACE_{PEEK,POKE}USR and PTRACE_{GET,SET}{REGS,FPREGS}
from arch_ptrace and let the rest be handled by generic code.  Define
PTRACE_SINGLEBLOCK to enable singleblock tracing.
[Geert] Not yet applicable for m68knommu

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