DwarfUnit: Share common constant value emission between APInts of small (<= 64 bit) and MCOperand immediates.

Doesn't seem a good reason to duplicate this code (it was more literally
duplicated prior to r208494, and while the dataN code /does/ actually
fire in this case, it doesn't seem necessary (and the DWARF standard
recommends using udata/sdata pervasively instead of dataN, so as to
indicate signedness of the values))