Mark runcmd with attribute noreturn, since it doesn't return

When compiling with gcc version 12, this change also avoids the
compiler warning "infinite recursion detected" for runcmd()

user/sh.c

@@ -52,6 +52,7 @@ struct backcmd {
 int fork1(void);  // Fork but panics on failure.
 void panic(char*);
 struct cmd *parsecmd(char*);
+void runcmd(struct cmd*) __attribute__((noreturn));
 
 // Execute cmd.  Never returns.
 void