efi: x86: Set the NX-compatibility flag in the PE header

Following Baskov Evgeniy's "Handle UEFI NX-restricted page tables"
patches, it's safe to set this compatibility flag to let loaders know
they don't need to make special accommodations for kernel to load if
pre-boot NX is enabled.

Signed-off-by: Peter Jones <pjones@redhat.com>
Link: https://lore.kernel.org/all/20220329184743.798513-1-pjones@redhat.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/x86/boot/header.S

@@ -163,7 +163,11 @@ extra_header_fields:
 	.long	0x200				# SizeOfHeaders
 	.long	0				# CheckSum
 	.word	IMAGE_SUBSYSTEM_EFI_APPLICATION	# Subsystem (EFI application)
+#ifdef CONFIG_DXE_MEM_ATTRIBUTES
+	.word	IMAGE_DLL_CHARACTERISTICS_NX_COMPAT	# DllCharacteristics
+#else
 	.word	0				# DllCharacteristics
+#endif
 #ifdef CONFIG_X86_32
 	.long	0				# SizeOfStackReserve
 	.long	0				# SizeOfStackCommit