avr: drop GNU toolchain dependency

- Use compiler-rt and picolibc instead of avr-libc.
  - Use ld.lld instead of avr-ld (or avr-gcc).

This makes it much easier to get started with TinyGo on AVR because
installing these extra tools (gcc-avr, avr-libc) can be a hassle.
It also opens the door for future improvements such as ThinLTO.

There is a code size increase but I think it's worth it in the long run.
The code size increase can hopefully be reduced with improvements to the
LLVM AVR backend and to compiler-rt.

targets/arduino-mega1280.json

@@ -2,7 +2,7 @@
     "inherits": ["atmega1280"],
     "build-tags": ["arduino_mega1280"],
     "ldflags": [
-        "-Wl,--defsym=_bootloader_size=4096"
+        "--defsym=_bootloader_size=4096"
     ],
     "flash-command":"avrdude -c arduino -b 57600 -p atmega1280 -P {port} -U flash:w:{hex}:i -v -D"
 }

targets/arduino-mega2560.json

@@ -2,7 +2,7 @@
     "inherits": ["atmega2560"],
     "build-tags": ["arduino_mega2560"],
     "ldflags": [
-        "-Wl,--defsym=_bootloader_size=8192"
+        "--defsym=_bootloader_size=8192"
     ],
     "flash-command":"avrdude -c wiring -b 115200 -p atmega2560 -P {port} -U flash:w:{hex}:i -v -D"
 }

targets/arduino-nano.json

@@ -2,8 +2,8 @@
 	"inherits": ["atmega328p"],
 	"build-tags": ["arduino_nano"],
 	"ldflags": [
-		"-Wl,--defsym=_bootloader_size=512",
-		"-Wl,--defsym=_stack_size=512"
+		"--defsym=_bootloader_size=512",
+		"--defsym=_stack_size=512"
 	],
 	"flash-command": "avrdude -c arduino -p atmega328p -b 57600 -P {port} -U flash:w:{hex}:i",
 	"emulator": "simavr -m atmega328p -f 16000000 {}"

targets/arduino.json

@@ -2,8 +2,8 @@
 	"inherits": ["atmega328p"],
 	"build-tags": ["arduino"],
 	"ldflags": [
-		"-Wl,--defsym=_bootloader_size=512",
-		"-Wl,--defsym=_stack_size=512"
+		"--defsym=_bootloader_size=512",
+		"--defsym=_stack_size=512"
 	],
 	"flash-command": "avrdude -c arduino -p atmega328p -P {port} -U flash:w:{hex}:i",
 	"serial-port": ["2341:0043", "2341:0001", "2a03:0043", "2341:0243"],

targets/atmega1280.json

@@ -4,8 +4,7 @@
     "build-tags": ["atmega1280", "atmega"],
     "serial": "uart",
     "ldflags": [
-        "-mmcu=avr51",
-        "-Wl,--defsym=_stack_size=512"
+        "--defsym=_stack_size=512"
     ],
     "linkerscript": "src/device/avr/atmega1280.ld",
     "extra-files": [

targets/atmega1284p.json

@@ -4,9 +4,8 @@
     "build-tags": ["atmega1284p", "atmega"],
     "serial": "uart",
     "ldflags": [
-        "-mmcu=avr51",
-        "-Wl,--defsym=_bootloader_size=0",
-        "-Wl,--defsym=_stack_size=512"
+        "--defsym=_bootloader_size=0",
+        "--defsym=_stack_size=512"
     ],
     "linkerscript": "src/device/avr/atmega1284p.ld",
     "extra-files": [

targets/atmega2560.json

@@ -4,8 +4,7 @@
     "build-tags": ["atmega2560", "atmega"],
     "serial": "uart",
     "ldflags": [
-        "-mmcu=avr6",
-        "-Wl,--defsym=_stack_size=512"
+        "--defsym=_stack_size=512"
     ],
     "linkerscript": "src/device/avr/atmega2560.ld",
     "extra-files": [

targets/atmega328p.json

@@ -3,9 +3,6 @@
 	"cpu": "atmega328p",
 	"build-tags": ["atmega328p", "atmega", "avr5"],
 	"serial": "uart",
-	"ldflags": [
-		"-mmcu=avr5"
-	],
 	"linkerscript": "src/device/avr/atmega328p.ld",
 	"extra-files": [
 		"targets/avr.S",

targets/attiny85.json

@@ -5,9 +5,6 @@
 	"cflags": [
 		"-D__AVR_ARCH__=25"
 	],
-	"ldflags": [
-		"-mmcu=avr25"
-	],
 	"linkerscript": "src/device/avr/attiny85.ld",
 	"extra-files": [
 		"targets/avr.S",

targets/avr.json

@@ -4,16 +4,17 @@
 	"goos": "linux",
 	"goarch": "arm",
 	"gc": "conservative",
-	"linker": "avr-gcc",
+	"linker": "ld.lld",
 	"scheduler": "none",
 	"rtlib": "compiler-rt",
+	"libc": "picolibc",
 	"default-stack-size": 256,
 	"cflags": [
 		"-Werror"
 	],
 	"ldflags": [
 		"-T", "targets/avr.ld",
-		"-Wl,--gc-sections"
+		"--gc-sections"
 	],
 	"extra-files": [
 		"src/internal/task/task_stack_avr.S",

targets/avr.ld

@@ -5,6 +5,8 @@ MEMORY
     RAM (xrw)       : ORIGIN = 0x800000 + __ram_start, LENGTH = __ram_size
 }
 
+ENTRY(__vector_RESET)
+
 SECTIONS
 {
     .text :
@@ -12,9 +14,11 @@ SECTIONS
         KEEP(*(.vectors))
         KEEP(*(.text.__vector_RESET))
         KEEP(*(.text.main)) /* main must follow the reset handler */
+        *(.text)
         *(.text.*)
         *(.progmem)
         *(.progmem.*)
+        . = ALIGN(16); /* needed with ld.lld for some reasoon */
     }
 
     .stack (NOLOAD) :

targets/digispark.json

@@ -2,8 +2,8 @@
 	"inherits": ["attiny85"],
 	"build-tags": ["digispark"],
 	"ldflags": [
-		"-Wl,--defsym=_bootloader_size=2180",
-		"-Wl,--defsym=_stack_size=128"
+		"--defsym=_bootloader_size=2180",
+		"--defsym=_stack_size=128"
 	],
 	"flash-command": "micronucleus --run {hex}",
 	"emulator": "simavr -m attiny85 -f 16000000 {}"