efi/zboot: Set forward edge CFI compat header flag if supported

Add some plumbing to the zboot EFI header generation to set the newly
introduced DllCharacteristicsEx flag associated with forward edge CFI
enforcement instructions (BTI on arm64, IBT on x86)

x86 does not currently uses the zboot infrastructure, so let's wire it
up only for arm64.

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