debug(core): make C impl of __fatal_error print to stderr

Previously, any C assertion or other case of __fatal_error would only
show the RSOD, but not print to emulator output.

That is (a) mildly annoying, and (b) would not work in the weird case in
Rust unit tests where graphics are not available.

core/embed/lib/error_handling.c

@@ -18,6 +18,9 @@
  */
 
 #include <stddef.h>
+#ifdef TREZOR_EMULATOR
+#include <stdio.h>
+#endif
 
 #include "common.h"
 #include "display.h"
@@ -65,6 +68,13 @@ void __attribute__((noreturn)) error_shutdown(const char *message) {
 
 void __attribute__((noreturn))
 __fatal_error(const char *msg, const char *file, int line) {
+#ifdef TREZOR_EMULATOR
+  fprintf(stderr, "FATAL ERROR: %s\n", msg);
+  if (file) {
+    fprintf(stderr, "file: %s:%d\n", file, line);
+  }
+  fflush(stderr);
+#endif
 #ifdef FANCY_FATAL_ERROR
   if (msg == NULL) {
     char buf[128] = {0};