Move ia64 efi.h to sys in preparation for amd64 UEFI support

  Prototypes specific to ia64 have been left in this file for now, under
  __ia64__, rather than moving them to a new header under sys/ia64.
  I anticipate that (some of) the corresponding functions will be shared
  by the amd64, arm64, i386, and ia64 architectures, and we can adjust
  this as EFI support on other than ia64 continues to develop.

Fix missed efi.h header change in r263815