x86: Cpu64 handling improvements

x86: Cpu64 handling improvements

First of all we want to also accumulate its reverse dependencies, such
that we can use them in cpu_flags_match(). This is in particular in
preparation of APX additions, such that e.g. BMI VEX-encoding templates
can become combined VEX/EVEX ones.

Once we have the reverse dependencies, we can further leverage them to
omit explicit "&x64" from any insn templates dealing with 64-bit-mode-
only ISA extensions. Besides helping readability for several insn
templates we already have, this will also help with what is going to be
added for APX (as all of the new templates would otherwise need to have
"&x64").

Note that rather than leaving a meaningless CPU_64_FLAGS (which is
unused anyway), its emitting is now also suppressed.