Add syscall tests and so harness

Add two tests performing syscalls, to show the running syscalls
themselves from within a restricted DDC environment works. I think this
is useful to have for future potential work (e.g., could we provide a
compartment-safe printing function, instead of crashing with `printf`,
if making `printf` be compatible proves hard).

Adds a harness to be able to run `so` files natively, without having to
go through the compartment library. Useful for debugging / differential
testing.

tests/simple_syscall_write.c

@@ -1,16 +1,15 @@
-#include <unistd.h>
-#include <string.h>
 #include <err.h>
+#include <string.h>
+#include <unistd.h>
 
 int
 main(void)
 {
-    char* buf = "Hello World!\n";
+    char *buf = "Hello World!\n";
     long int sc_write = syscall(4, STDOUT_FILENO, buf, strlen(buf));
     if (sc_write == -1)
     {
         err(1, "Error calling `syscall`:");
     }
     return 0;
 }
-