About mpc8540 problem

[Aubrey]

Hi, I'm porting the u-boot to the MPC8540SK board. And Now
I get a problem.
When I run to the "printf" function, unexpected interruption
occurs. Then I trace into the assemble file. I found that something
wrong with the function "va_start","va_end". I know that these defines 
may be include by the stdarg.h file. But in the u-boot file tree, I
can not find this file. Does u-boot use the file of my host system?
I don't know how to solve this problem. 
Any comment and suggestion will be welcome!
thank you!