Fix ARM64 relocation function unused attrs

Similar to other architectures.

src/system/boot/platform/efi/arch/arm64/relocation_func.cpp

@@ -40,8 +40,8 @@
 #include <elf.h>
 
 extern "C" efi_status _relocate (long ldbase, Elf64_Dyn *dyn,
-		      efi_handle image EFI_UNUSED,
-		      efi_system_table *systab EFI_UNUSED)
+		      efi_handle image __attribute__((__unused__)),
+		      efi_system_table *systab __attribute__((__unused__)))
 {
 	long relsz = 0, relent = 0;
 	Elf64_Rela *rel = 0;