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.

builder/builtins.go

@@ -162,6 +162,15 @@ var aeabiBuiltins = []string{
 	"udivmodsi4.c",
 }
 
+var avrBuiltins = []string{
+	"avr/divmodhi4.S",
+	"avr/divmodqi4.S",
+	"avr/mulhi3.S",
+	"avr/mulqi3.S",
+	"avr/udivmodhi4.S",
+	"avr/udivmodqi4.S",
+}
+
 // CompilerRT is a library with symbols required by programs compiled with LLVM.
 // These symbols are for operations that cannot be emitted with a single
 // instruction or a short sequence of instructions for that target.
@@ -186,6 +195,9 @@ var CompilerRT = Library{
 		if strings.HasPrefix(target, "arm") || strings.HasPrefix(target, "thumb") {
 			builtins = append(builtins, aeabiBuiltins...)
 		}
+		if strings.HasPrefix(target, "avr") {
+			builtins = append(builtins, avrBuiltins...)
+		}
 		return builtins, nil
 	},
 }

builder/picolibc.go

@@ -78,7 +78,7 @@ var picolibcSources = []string{
 	"libc/tinystdio/fputc.c",
 	"libc/tinystdio/fputs.c",
 	"libc/tinystdio/fread.c",
-	"libc/tinystdio/freopen.c",
+	//"libc/tinystdio/freopen.c", // crashes with AVR, see: https://github.com/picolibc/picolibc/pull/369
 	"libc/tinystdio/fscanf.c",
 	"libc/tinystdio/fseek.c",
 	"libc/tinystdio/fseeko.c",