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.

src/runtime/arch_avr.go

@@ -37,3 +37,39 @@ func procPin() {
 func procUnpin() {
 	interrupt.Restore(procPinnedMask)
 }
+
+// The following functions are workarounds for things missing in compiler-rt.
+// They will likely need special assembly implementations.
+
+//export __mulsi3
+func __mulsi3(a, b uint32) uint32 {
+	var r uint32
+	for a != 0 {
+		if a&1 != 0 {
+			r += b
+		}
+		a >>= 1
+		b <<= 1
+	}
+	return r
+}
+
+//export __divsi3
+func __divsi3(a, b int32) int32
+
+//export __udivsi3
+func __udivsi3(a, b uint32) uint32
+
+//export __divmodsi4
+func __divmodsi4(a, b int32) uint64 {
+	d := __divsi3(a, b)
+	rem := a - (d * b)
+	return uint64(uint32(d)) | uint64(uint32(rem))<<32
+}
+
+//export __udivmodsi4
+func __udivmodsi4(a, b uint32) uint64 {
+	d := __udivsi3(a, b)
+	rem := a - (d * b)
+	return uint64(d) | uint64(rem)<<32
+}