Support printing the empty list

scm/inc/compiler.scm

  (emit "movl $~a, %eax" (immediate-rep x))
  (emit "ret"))
(compile-program 42)
(compile-program '())

scm/inc/driver.c

#define fixnum_tag   0 // 00
#define fixnum_shift 2
#define empty_list   47 // 00101111
int scheme_entry();
int main(int argc, char **argv) {
	int val = scheme_entry();
	if ((val & fixnum_mask) == fixnum_tag) {
		printf("%d\n", val >> fixnum_shift);
	} else if (val == empty_list) {
		printf("()\n");
	} else {
		printf("Error: unhandled value: %d\n", val);
		return 1;