[fixed-point] Support constant folding

[Eric Botcazou]

> 	* fixed_value.h (fconst0): Declare.
> 	(fixed_arithmetic, fixed_compare, fixed_arithmetic2): Declare.
> 	(FIXED_VALUE_NEGATE): Define.
> 	* fixed_value.c (do_fixed_add, do_fixed_multiply, do_fixed_divide,
> 	do_fixed_shift): Declare.
> 	(fixed_from_string): Sign or zero extend the value.
> 	(do_fixed_add, do_fixed_multiply, do_fixed_divide, do_fixed_shift,
> 	fixed_arithmetic, fixed_arithmetic2, fixed_compare): New functions.

I don't think we want underscores in filenames, see the existing files.

-- 
Eric Botcazou