preproc: add %abs(), modify %num(), refactor

Add the %abs() function, to produce the absolute value as an
always-positive decimal constant.

Change the order of the arguments for %num().

Refactor the handling of optional arguments, to reduce the amount of
redundant code. This is currently only used for builtin functions, but
might be extended in the future.

Signed-off-by: H. Peter Anvin <[email protected]>

Author: H. Peter Anvin