ART: Refactor long ops in quick compiler

Make GenArithOpLong virtual. Let the implementation in gen_common be
very basic, without instruction-set checks, and meant as a fall-back.
Backends should implement and dispatch to code for better implementations.
This allows to remove the GenXXXLong virtual methods from Mir2Lir, and
clean up the backends (especially removing some LOG(FATAL) implementations).

Change-Id: I6366443c0c325c1999582d281608b4fa229343cf