x86: Correct get_sp() implementation for 64-bit

Use an assembler implementation as is done for i386, so that the results
are equivalent for i386 and x86_64.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

arch/x86/lib/bootm.c

@@ -258,7 +258,7 @@ static ulong get_sp(void)
 	ulong ret;
 
 #if CONFIG_IS_ENABLED(X86_64)
-	ret = gd->start_addr_sp;
+	asm("mov %%rsp, %0" : "=r"(ret) : );
 #else
 	asm("mov %%esp, %0" : "=r"(ret) : );
 #endif