target/m68k: add monitor.c

This allows to use registers content in the monitor.

Example:

 BEFORE:
  (qemu) print $d0
  unknown register

 AFTER:
  (qemu) print $d0
  0
  (qemu) print $sr
  0x2000
  (qemu) x/10i $pc
  0x40010a2a:  movew %sr,%d0
  0x40010a2c:  oril #1792,%d0
  0x40010a32:  movew %d0,%sr
  0x40010a34:  movel %a0@,%d0
  0x40010a36:  btst #3,%d0
  0x40010a3a:  beqs 0x40010a26
  0x40010a3c:  movew %sr,%d0
  0x40010a3e:  andil #63743,%d0
  0x40010a44:  movew %d0,%sr
  0x40010a46:  rts

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20171221083057.17942-1-laurent@vivier.eu>