dink/fbneo - June 24, 2020

i86 NEG (f6 18, f7 18) opcode fix.

Abstract: NEG has a different operation on NEC v20 cpu's.  When v20 was
backported to i86 in this commit:

https://github.com/dinkc64/mame/commit/d41f2ae4f8a7d6e5c4afef51a52ccf1a89874c8f#diff-ddc58133e6669a4d92c93fa1e4ec1cee

.. It cause issue with the game Krull, enemies would spawn yet be invisible
because the game depends on the i86-style NEG.

i86 NEG changes all the flags that would result from a SUBB/SUBW.
v20 NEG changes only the carry flag, before the operation is even made. ref:
NEC vXX manual:

https://www.ceibo.com/eng/datasheets/NEC-V20-V30-Users-Manual.pdf

p.s. please credit dink/fbneo for this if used.
