[coreboot] Recent v2 abuild breakage

[Carl-Daniel Hailfinger]

Hi!

The recent v2 abuild breakage we're seeing since the move to a new
massively multiprocessor server may have been caused way back by r3564.
AFAICS it seems the dependency rules on romcc cause it to be compiled
twice if certain race conditions happen.
The v2 build system is sort of a mystery to me, so the following may or
may not work. I think I can fix this by compiling romcc into a temp file
and moving the temp file in the right place afterwards. That makes sure
no two compiler processes try to output to the same file at the same
time and it also makes sure that the romcc binary will always be runnable.

Any hints or better fixes are appreciated.

Regards,
Carl-Daniel