cmd: bootvx: Add asmlinkage to the VxWorks x86 entry

VxWorks on x86 uses stack to pass parameters.

Reported-by: Jian Luo <jian.luo4@boschrexroth.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>