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>

Makefile

@@ -1100,7 +1100,7 @@ u-boot-payload.lds: $(LDSCRIPT_EFI) FORCE
 # Rule to link the EFI payload which contains a stub and a U-Boot binary
 quiet_cmd_u-boot_payload ?= LD      $@
       cmd_u-boot_payload ?= $(LD) $(LDFLAGS_EFI_PAYLOAD) -o $@ \
-      -T u-boot-payload.lds \
+      -T u-boot-payload.lds arch/x86/cpu/call32.o \
       lib/efi/efi.o lib/efi/efi_stub.o u-boot-dtb.bin.o \
       $(addprefix arch/$(ARCH)/lib/efi/,$(EFISTUB))