execute_alu: make branch destination calculation optional

All pipeline implementations don't wish to do the
branch destination calculation in execute stage,
so to avoid inserting redundant logic for them,
make this optional by adding a parameter.
The parameter defaults to "old" behaviour.
cappuccino/cpu: disable branch destination calculation in ALU
execute_alu: make alu_result default to adder_result

The comment about "jump reg instructions" is clearly
wrong, since the jump reg instructions assigns alu_result
to adder_result just above this comment.
Also, remove the comment-out around OPC_ADDC/ADD/SUB,
the end result is the same, but less confusing.