boot: Add EFISTUB and refactor boot protocols

boot/efi/x86_64/efi_arch.h

@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: ISC
+
+#pragma once
+
+#include <stdint.h>
+
+#define EFIAPI __attribute__((ms_abi))
+
+constexpr uint16_t EFI_ARCH_PE_MACHINE = 0x8664;