cpu: native: Add [v]fprintf to syscalls

External packages that may use fprintf(stderr, ...) for logging will
link directly to the libc version of it, and for some reason that
results in the application crashing.

cpu/native/syscalls.c

@@ -289,21 +289,33 @@ int printf(const char *format, ...)
 {
     int r;
     va_list argp;
-    char *m;
 
     va_start(argp, format);
-    if ((m = make_message(format, argp)) == NULL) {
-        err(EXIT_FAILURE, "malloc");
-    }
-    r = _native_write(STDOUT_FILENO, m, strlen(m));
+    r = vfprintf(stdout, format, argp);
     va_end(argp);
-    free(m);
 
     return r;
 }
 
 
 int vprintf(const char *format, va_list argp)
+{
+    return vfprintf(stdout, format, argp);
+}
+
+int fprintf(FILE *fp, const char *format, ...)
+{
+    int r;
+    va_list argp;
+
+    va_start(argp, format);
+    r = vfprintf(fp, format, argp);
+    va_end(argp);
+
+    return r;
+}
+
+int vfprintf(FILE *fp, const char *format, va_list argp)
 {
     int r;
     char *m;
@@ -311,7 +323,7 @@ int vprintf(const char *format, va_list argp)
     if ((m = make_message(format, argp)) == NULL) {
         err(EXIT_FAILURE, "malloc");
     }
-    r = _native_write(STDOUT_FILENO, m, strlen(m));
+    r = _native_write(fileno(fp), m, strlen(m));
     free(m);
 
     return r;