wasm: execute an 'unreachable' trap inst on panic

This already happened when directly calling panic() but not with some
runtime panics like accessing a slice out of bounds.

src/runtime/panic.go

@@ -1,5 +1,10 @@
 package runtime
 
+// trap is a compiler hint that this function cannot be executed. It is
+// translated into either a trap instruction or a call to abort().
+//go:linkname trap llvm.trap
+func trap()
+
 // Builtin function panic(msg), used as a compiler intrinsic.
 func _panic(message interface{}) {
 	printstring("panic: ")

src/runtime/runtime_wasm.go

@@ -49,6 +49,7 @@ func align(ptr uintptr) uintptr {
 	return (ptr + 3) &^ 3
 }
 
+// Abort executes the wasm 'unreachable' instruction.
 func abort() {
-	// TODO
+	trap()
 }