RFA: Fix calculation of size of builtin setjmp buffer

[Eric Botcazou]

> But, fails whenever the size of the mode of the save area is bigger than a
> certain amount…  On my port, the size taken up by the  save area is large
> enough to cause this to fail.  :-(

That's a bit unexpected, why do you need so big a save area exactly?  The only 
architecture for which this doesn't work is the IA-64, which is a very special 
beast...  In this case, the way out is to define DONT_USE_BUILTIN_SETJMP and 
JMP_BUF_SIZE to the needed size.

-- 
Eric Botcazou