Add riscv target, enhance arm target

test/mongoose_custom.c

@@ -0,0 +1,40 @@
+#include "mongoose.h"
+
+int usleep(useconds_t us) {
+  for (useconds_t i = 0; i < us * 99; i++) asm("nop");
+  return 0;
+}
+
+int clock_gettime(clockid_t clock_id, struct timespec *tp) {
+  (void) clock_id;
+  memset(tp, 0, sizeof(*tp));
+  return 0;
+}
+
+struct mg_connection *mg_connect(struct mg_mgr *mgr, const char *url,
+                                 mg_event_handler_t fn, void *fn_data) {
+  (void) mgr, (void) url, (void) fn, (void) fn_data;
+  return NULL;
+}
+
+void mg_connect_resolved(struct mg_connection *c) {
+  (void) c;
+}
+
+struct mg_connection *mg_listen(struct mg_mgr *mgr, const char *url,
+                                mg_event_handler_t fn, void *fn_data) {
+  (void) mgr, (void) url, (void) fn, (void) fn_data;
+  return NULL;
+}
+
+void mg_mgr_poll(struct mg_mgr *mgr, int ms) {
+  (void) mgr, (void) ms;
+}
+
+bool mg_send(struct mg_connection *c, const void *buf, size_t len) {
+  (void) c, (void) buf, (void) len;
+  return 0;
+}
+
+void _fini(void) {
+}