checkasm: Add --list-cpuflags option

Instead of adding yet another boolean flag this introduces an enum for signaling the current "run mode".

Printing cpuflags which are not supported on the current system could be useful if those flags are ifdef'ed out from the cpus[] table when unsupported by the compiler/assembler (e.g. !1616 (merged)) in order to distinguish between "not supported by this toolchain" and "not supported by this cpu".