arm64: efi: Set NX compat flag in PE/COFF header

The PE/COFF header has a NX compat flag which informs the firmware that
the application does not rely on memory regions being mapped with both
executable and writable permissions at the same time.

This is typically used by the firmware to decide whether it can set the
NX attribute on all allocations it returns, but going forward, it may be
used to enforce a policy that only permits applications with the NX flag
set to be loaded to begin wiht in some configurations, e.g., when Secure
Boot is in effect.

Even though the arm64 version of the EFI stub may relocate the kernel
before executing it, it always did so after disabling the MMU, and so we
were always in line with what the NX compat flag conveys, we just never
bothered to set it.

So let's set the flag now.

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

arch/arm64/kernel/efi-header.S

@@ -66,7 +66,7 @@
 	.long	.Lefi_header_end - .L_head		// SizeOfHeaders
 	.long	0					// CheckSum
 	.short	IMAGE_SUBSYSTEM_EFI_APPLICATION		// Subsystem
-	.short	0					// DllCharacteristics
+	.short	IMAGE_DLL_CHARACTERISTICS_NX_COMPAT	// DllCharacteristics
 	.quad	0					// SizeOfStackReserve
 	.quad	0					// SizeOfStackCommit
 	.quad	0					// SizeOfHeapReserve