Work around linkage bug cross-compiling from x86_64-apple-darwin to i686-apple-darwin

The correct opendir/readdir to use appear to be the 64-bit versions called
opendir$INODE64, etc. but for some reason I can't get them to link properly
on i686. Putting them in librustrt and making gcc figure it out works.
This mystery will have to wait for another day.

src/rt/rust_builtin.cpp

@@ -818,6 +818,33 @@ rust_dec_kernel_live_count() {
     task->kernel->dec_live_count();
 }
 
+#ifndef _WIN32
+#include <sys/types.h>
+#include <dirent.h>
+
+extern "C" DIR*
+rust_opendir(char *dirname) {
+    return opendir(dirname);
+}
+
+extern "C" dirent*
+rust_readdir(DIR *dirp) {
+    return readdir(dirp);
+}
+
+#else
+
+extern "C" void
+rust_opendir() {
+}
+
+extern "C" void
+rust_readdir() {
+}
+
+#endif
+
+
 //
 // Local Variables:
 // mode: C++