Fix --help output for options without a short flag

Previously --help printed the following:
```
         -V      --no-trace
         -�      --trace-output
         -l      --inst-limit
```
With the new change it is:
```
         -V      --no-trace
                 --trace-output
         -l      --inst-limit
```

c_emulator/riscv_sim.c

@@ -1,3 +1,4 @@
+#include <ctype.h>
 #include <getopt.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -156,7 +157,10 @@ static void print_usage(const char *argv0, int ec)
 #endif
   struct option *opt = options;
   while (opt->name) {
-    fprintf(stdout, "\t -%c\t --%s\n", (char)opt->val, opt->name);
+    if (isprint(opt->val))
+      fprintf(stdout, "\t -%c\t --%s\n", (char)opt->val, opt->name);
+    else
+      fprintf(stdout, "\t   \t --%s\n", opt->name);
     opt++;
   }
   exit(ec);