parsergen: pass 'config' in to 'reduce' function.

parsergen: pass 'config' in to 'reduce' function.

As we only support synthesise attributes and no inherited attributes,
we have no way for the reduce functions to access any context (such
as building a table of variables) except via global variables (yuck).

So pass the 'context' pointer through.  The main program can embed
this in a larger structure which contains relevant context, and
the reduce functions can find that using pointer manipulation.

Signed-off-by: NeilBrown <neil@brown.name>