m68k: ptrace fixes

This fixes the following issues in ptrace:

- when single stepping into the signal handler stop at the first insn of
  the handler
- handle non-zero stkadj when accessing pc and sr in ptregs
- correctly handle PT_SR in PTRACE_POKEUSR
- report -EIO when trying to read unknown offset in PTRACE_PEEKUSR

Additionally, the handling of the special case that PT_SR accesses a 16
bit word instead of a 32 bit word has been moved into get_reg/put_reg.

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