Sync output devices on exit

Mbed retarget does an `fflush` on stdout and stderr on exit - this
flushes the C library buffers (if it is buffering), but doesn't
flush any device buffers (eg UARTSerial's TX buffer). Add
sync() calls to the output device to do this.

platform/mbed_retarget.cpp

@@ -1250,6 +1250,8 @@ extern "C" void exit(int return_code)
 #if MBED_CONF_PLATFORM_STDIO_FLUSH_AT_EXIT
     fflush(stdout);
     fflush(stderr);
+    fsync(STDOUT_FILENO);
+    fsync(STDERR_FILENO);
 #endif
 #endif