arm: async uart writes & fixes

arm/stdlib.cc

@@ -12,9 +12,9 @@ extern unsigned char _heap_begin, _heap_end;
 namespace {
 void LogStats(unsigned char* heap) {
     char number[] = "00000000\r\n";
-    UartSend("Program break now at 0x");
+    UartWriteBlocking("Program break now at 0x");
     itoa(reinterpret_cast<int>(heap), number);
-    UartSend(number);
+    UartWriteBlocking(number);
 }
 }  // namespace
 
@@ -22,7 +22,7 @@ extern "C" void* _sbrk(int increment) {
     static unsigned char* heap = &_heap_begin;
     unsigned char* prev_heap = heap;
     if (heap + increment >= &_heap_end) {
-        UartSend("Heap overflow!\r\n");
+        UartWriteBlocking("Heap overflow!\r\n");
         return reinterpret_cast<void*>(-1);
     }
     heap += increment;