x86: Use default stack boundary alignment

At present U-Boot x86 build is using -mpreferred-stack-boundary=2
which is 4 bytes stack boundary alignment. With 64-bit U-Boot, the
minimal required stack boundary alignment is 16 bytes.

If -mpreferred-stack-boundary is not specified, the default is 4
(16 bytes). Switch to use the default one.

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