target/avr: Add support for disassembling via option '-d in_asm'

Provide function disassembles executed instruction when '-d in_asm' is
provided.

Example:

$ qemu-system-avr -bios free-rtos/Demo/AVR_ATMega2560_GCC/demo.elf -d in_asm
    ...
    IN:
    0x0000014a:  CALL      0x3808

    IN: main
    0x00003808:  CALL      0x4b4

    IN: vParTestInitialise
    0x000004b4:  LDI       r24, 255
    0x000004b6:  STS       r24, 0
    0x000004b8:  MULS      r16, r20
    0x000004ba:  OUT       $1, r24
    0x000004bc:  LDS       r24, 0
    0x000004be:  MULS      r16, r20
    0x000004c0:  OUT       $2, r24
    0x000004c2:  RET
    ...

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Suggested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Suggested-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Signed-off-by: Michael Rolnik <mrolnik@gmail.com>
[rth: Fix spacing and const mnemonic arrays]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Aleksandar Markovic <aleksandar.m.mail@gmail.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>
Message-Id: <20200705140315.260514-19-huth@tuxfamily.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>