Fix regressions affecting compilation

This fixes two regressions, caused by #1167 and #1191 .

Marlin/SdFatUtil.cpp

@@ -26,21 +26,24 @@
 /** Amount of free RAM
  * \return The number of free bytes.
  */
+#ifdef __arm__
+extern "C" char* sbrk(int incr);
 int SdFatUtil::FreeRam() {
-  extern int  __bss_end;
-  extern int* __brkval;
-  int free_memory;
-  if (reinterpret_cast<int>(__brkval) == 0) {
-    // if no heap use from end of bss section
-    free_memory = reinterpret_cast<int>(&free_memory)
-                  - reinterpret_cast<int>(&__bss_end);
-  } else {
-    // use from top of stack to heap
-    free_memory = reinterpret_cast<int>(&free_memory)
-                  - reinterpret_cast<int>(__brkval);
-  }
-  return free_memory;
+  char top;
+  return &top - reinterpret_cast<char*>(sbrk(0));
 }
+#else  // __arm__
+extern char *__brkval;
+extern char __bss_end;
+/** Amount of free RAM
+ * \return The number of free bytes.
+ */
+int SdFatUtil::FreeRam() {
+  char top;
+  return __brkval ? &top - __brkval : &top - &__bss_end;
+}
+#endif  // __arm
+
 //------------------------------------------------------------------------------
 /** %Print a string in flash memory.
  *