[ARM] cs_reg_name() might return system reg name instead of banked reg name

[Rot127]

As explained in #2116 system and banked registers can sometimes not differentiated by their encoding (e.g. 2057 = psp (sysreg) = r9_fiq (bankedreg).

cs_reg_name() will always return the system register name first (because this is most likely asked for).

Nonetheless it would be nice to either expose a new function or extend cs_reg_name with a flag.