Prep dynamic linker.

libc/arch-nacl/syscalls/mkdir.c

Index: libc/arch-nacl/syscalls/mkdir.c
diff --git a/libc/arch-nacl/syscalls/mkdir.c b/libc/arch-nacl/syscalls/mkdir.c
index 1946f6f3eeb827ca40d08dd4bfb24483be1b18e7..597c47300049687dbcf31672d725b16bbca938b1 100644
--- a/libc/arch-nacl/syscalls/mkdir.c
+++ b/libc/arch-nacl/syscalls/mkdir.c
@@ -6,11 +6,16 @@
 #include <sys/stat.h>
 
 int mkdir(const char *pathname, mode_t mode) {
-  size_t nwrite;
-  int result = nacl_irt_mkdir(pathname, mode);
-  if (result != 0) {
-    errno = result;
-    return -1;
+  if (__nacl_irt_mkdir) {
+    size_t nwrite;
+    int result = __nacl_irt_mkdir(pathname, mode);
+    if (result != 0) {
+      errno = result;
+      return -1;
+    }
+    return 0;
   }
+  errno = ENOSYS;
+  return -1;
 }