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

The IMAGE_DLLCHARACTERISTICS_NX_COMPAT informs the firmware that the
EFI binary does not rely on pages that are both executable and
writable.

The flag is used by some distro versions of GRUB to decide if the EFI
binary may be executed.

As the Linux kernel neither has RWX sections nor needs RWX pages for
relocation we should set the flag.

Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Fixes: cb7d2dd ("RISC-V: Add PE/COFF header for EFI stub")
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240929140233.211800-1-heinrich.schuchardt@canonical.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: xypron

arch/riscv/kernel/efi-header.S

@@ -64,7 +64,7 @@ extra_header_fields:
 	.long	efi_header_end - _start			// 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