[x86_64] Add support for compilation as an x86_64 binary

Currently the only supported platform for x86_64 is EFI.

Building an EFI64 gPXE requires a version of gcc that supports
__attribute__((ms_abi)).  This currently means a development build of
gcc; the feature should be present when gcc 4.4 is released.

In the meantime; you can grab a suitable gcc tree from

  git://git.etherboot.org/scm/people/mcb30/gcc/.git

src/arch/x86/include/bits/pci_io.h

@@ -0,0 +1,13 @@
+#ifndef _BITS_PCI_IO_H
+#define _BITS_PCI_IO_H
+
+/** @file
+ *
+ * i386-specific PCI I/O API implementations
+ *
+ */
+
+#include <gpxe/pcibios.h>
+#include <gpxe/pcidirect.h>
+
+#endif /* _BITS_PCI_IO_H */