target/avr: CPU class: Add GDB support

target/avr: CPU class: Add GDB support

This includes GDB hooks for reading from wnd wrtiting to AVR
registers, and xml register definition file as well.

[AM: Split a larger AVR introduction patch into logical units]
Suggested-by: Aleksandar Markovic <[email protected]>
Co-developed-by: Michael Rolnik <[email protected]>
Co-developed-by: Sarah Harris <[email protected]>
Signed-off-by: Michael Rolnik <[email protected]>
Signed-off-by: Sarah Harris <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Signed-off-by: Aleksandar Markovic <[email protected]>
Acked-by: Igor Mammedov <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
[thuth: Fixed avr_cpu_gdb_read_register() parameter]
Signed-off-by: Thomas Huth <[email protected]>
Message-Id: <20200705140315[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>