[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_64/include/bits/uaccess.h

@@ -0,0 +1,10 @@
+#ifndef _BITS_UACCESS_H
+#define _BITS_UACCESS_H
+
+/** @file
+ *
+ * x86_64-specific user access API implementations
+ *
+ */
+
+#endif /* _BITS_UACCESS_H */