arm: async echo app

arm/itoa.h

@@ -0,0 +1,13 @@
+#pragma once
+
+// out must be at least 8 bytes long
+inline void itoa(int val, char* out) {
+    for (int i = 0; i < 8; i++) {
+        uint8_t digit = (val >> (28 - 4 * i)) & 0xf;
+        if (digit < 0xa) {
+            out[i] = '0' + digit;
+        } else {
+            out[i] = 'a' + digit - 0xa;
+        }
+    }
+}