fix - first-ever attempt at running synthesis.tcl would fail; now
it will work too.
More details:
-since the RCA inside the PC_ACC component was replaced with a p4_adder,
the p4_adder needs to be analyzed/compiled BEFORE the PC_ACC;
-but with the previous analysis ordering, the first
time you'd run synthesis.tcl it would complain and fail, because it
would try to analyze PC_ACC before analyzing the p4_adder.
-solution: leave p4_adder coude where it was (so that people reading
source code of arithmetic unit can still find it easily), but also place
symlinks in the top-level directory, so that it gets analyzed first
(more or less at the same time as the old RCA).
-note that the actual p4_adder source (8 files) must now be kept out
of the synthesis.tcl files list, since p4_adder is already synthesized
once when analyzing its symlinks.