x86: unify CPU flag on/off processing

There's no need for the arbitrary special "unknown" token: Simply
recognize the leading ~ and process everything else the same, merely
recording whether to set individual fields to 1 or 0.

While there exclude CpuIAMCU from CPU_UNKNOWN_FLAGS - CPU_IAMCU_FLAGS
override cpu_arch_flags anyway when -march=iamcu is passed, and there's
no reason to have the stray flag set even if no insn actually is keyed
to it.