Refactor process_args to return the argv index instead of the value

This makes it possible for a follow-up commit to add logic that allows
loading more than one ELF file (e.g. M-mode firmware and S-mode kernel).

c_emulator/riscv_sim.c

@@ -226,7 +226,11 @@ static void read_dtb(const char *path)
   fprintf(stdout, "Read %zd bytes of DTB from %s.\n", dtb_len, path);
 }
 
-char *process_args(int argc, char **argv)
+/**
+ * Parses the command line arguments and returns the argv index for the ELF file
+ * that should be loaded.
+ */
+static int process_args(int argc, char **argv)
 {
   int c;
   uint64_t ram_size = 0;
@@ -388,7 +392,7 @@ char *process_args(int argc, char **argv)
   if (!rvfi_dii)
 #endif
     fprintf(stdout, "Running file %s.\n", argv[optind]);
-  return argv[optind];
+  return optind;
 }
 
 void check_elf(bool is32bit)
@@ -1044,7 +1048,8 @@ int main(int argc, char **argv)
   // Initialize model so that we can check or report its architecture.
   preinit_sail();
 
-  char *file = process_args(argc, argv);
+  int files_start = process_args(argc, argv);
+  char *file = argv[files_start];
   init_logs();
 
   if (gettimeofday(&init_start, NULL) < 0) {