Add Arduino M0 Pro support

[gandarez]

This is an ongoing PR.

It wash heavily based on boards.txt from Arduino repo.

It fails to build with:

tinygo flash -target arduino-m0-pro ./main.go

Avrdude version 8.0
Copyright see https://github.com/avrdudes/avrdude/blob/main/AUTHORS

System wide configuration file is /opt/homebrew/etc/avrdude.conf
User configuration file <User>/.avrduderc does not exist
Warning: system wide configuration file version (7.3)
does not match Avrdude build version (8.0)

Using port            : /dev/cu.usbmodem004F1
Using programmer      : stk500v2
Setting baud rate     : 57600
AVR part              : ATmega2560
Programming modes     : SPM, ISP, HVPP, JTAG
Programmer type       : STK500V2
Description           : Atmel STK500 version 2.x firmware
Programmer model      : AVRISP
HW version            : 3
FW Version Controller : 4.05
Vtarget               : 0.0 V
SCK period            : 0.1 us

AVR device initialized and ready to accept instructions
Device signature = 1E 98 01 (ATmega2560)
Auto-erasing chip as flash memory needs programming (-U flash:w:...)
specify the -D option to disable this feature
Erased chip
Reading 6184 bytes for flash from input file main.hex
in 1 section [0x2000, 0x3827]: 25 pages and 216 pad bytes
Writing 6184 bytes to flash
Writing | ################################################## | 100% 0.21 s 
Reading | ################################################## | 100% 0.18 s 
Warning: flash verification mismatch
  device 0xff != input 0x00 at addr 0x2000 (error)
  device 0xff != input 0x08 at addr 0x2001 (error)
  device 0xff != input 0x00 at addr 0x2002 (error)
  device 0xff != input 0x20 at addr 0x2003 (error)
  device 0xff != input 0x19 at addr 0x2004 (error)
  device 0xff != input 0x32 at addr 0x2005 (error)
  device 0xff != input 0x00 at addr 0x2006 (error)
  device 0xff != input 0x00 at addr 0x2007 (error)
  device 0xff != input 0x37 at addr 0x2008 (error)
  device 0xff != input 0x21 at addr 0x2009 (error)
  suppressing further verification errors
Error: flash verification mismatch

Avrdude done.  Thank you.
failed to flash /var/folders/m4/q3fvlgld52d2m3k1wmmvb1pm0000gn/T/tinygo2035335312/main.hex: exit status 1