efi/pe: Import new BTI/IBT header flags from the spec

The latest version of your favorite fork of the PE/COFF spec includes a
new type of header flag that is intended to be used in the context of
EFI firmware to indicate to the image loader that the executable regions
of an image can be mapped with BTI/IBT enforcement enabled.

So let's import these definitions so we can use them in subsequent
patches.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>