Add mt-helloworld example

tests/hello.c

@@ -1,6 +1,10 @@
 #include <stdio.h>
+#include "encoding.h"
+#include "marchid.h"
 
 int main(void) {
-  printf("Hello world\n");
+  uint64_t marchid = read_csr(marchid);
+  const char* march = get_march(marchid);
+  printf("Hello world from core 0, a %s\n", march);
   return 0;
 }