CORE-R6.xpr timing closure issues

[sy2002]

For the time being, CORE-R5.xpr and CORE-R6.xpr should be identical. Yet, CORE-R6.xpr does not close timing (see screenshots), while `CORE-R5.xpr? does.

With this commit https://github.com/MJoergen/C64MEGA65/commit/81749eaba8db80b3f628f8959b207e29a0255589, we achieve timing closure on R6.

We need to investigate, if CORE-R6.xpr has a bug and/or how it is different from CORE-R5.xpr or why we don't get timing closure.

For the V5.1 release we did not use the bitstream generated by CORE-R6.xpr but copied the bitstream generated by CORE-R5.xpr, because for the time being both platforms are identical when it comes to the C64 core.