all: add WebAssembly backend

src/runtime/arch_avr.go

@@ -2,6 +2,10 @@
 
 package runtime
 
+import (
+	"unsafe"
+)
+
 const GOARCH = "avr"
 
 // The length type used inside strings and slices.
@@ -9,3 +13,6 @@ type lenType uint16
 
 // The bitness of the CPU (e.g. 8, 32, 64).
 const TargetBits = 8
+
+//go:extern _heap_start
+var heapStart unsafe.Pointer