efi: Add 64-bit payload support

Most EFI implementations use 64-bit. Add a way to build U-Boot as a 64-bit
EFI payload. The payload unpacks a (32-bit) U-Boot and starts it. This can
be enabled for x86 boards at present.

Signed-off-by: Simon Glass <sjg@chromium.org>
Improvements to how the payload is built:
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

include/efi.h

@@ -18,6 +18,13 @@
 #include <linux/string.h>
 #include <linux/types.h>
 
+#ifdef CONFIG_EFI_STUB_64BIT
+/* EFI uses the Microsoft ABI which is not the default for GCC */
+#define EFIAPI __attribute__((ms_abi))
+#else
+#define EFIAPI
+#endif
+
 struct efi_device_path;
 
 #define EFI_SUCCESS		0