change default zeta_C compiler to gcc & complete 10815.c

zeta_C/run.sh

@@ -1,4 +1,4 @@
 #!/bin/bash
 if [ -z "$*" ]; then echo "No args"; exit 0; fi
-clang-18 -std=c11 $1
+gcc -std=c11 $1
 ./a.out < stdin.txt