x86: Add 64-bit start-up code

Add code to start up U-Boot in 64-bit mode. It is fairly simple since we are
running from RAM and SPL has done the low-level init.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/cpu/Makefile b/arch/x86/cpu/Makefile
index fd81310..97b26b0 100644
--- a/arch/x86/cpu/Makefile
+++ b/arch/x86/cpu/Makefile
@@ -8,7 +8,11 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
+ifeq ($(CONFIG_$(SPL_)X86_64),y)
+extra-y	= start64.o
+else
 extra-y	= start.o
+endif
 extra-$(CONFIG_$(SPL_)X86_16BIT_INIT) += resetvec.o start16.o
 obj-y	+= interrupts.o cpu.o cpu_x86.o call64.o setjmp.o