Restructure the codegen to make architectural depedency explicit.

Restructure the codegen to make architectural depedency explicit.

The original Codegen.c is broken into three components:

- CodegenCommon.c (arch-independend)
- CodegenFactory.c (Thumb1/2 dependent)
- CodegenDriver.c (Dalvik dependent)

For the Thumb/Thumb2 directories, each contain the followin three files:

- Factory.c (low-level routines for instruction selections)
- Gen.c     (invoke the ISA-specific instruction selection routines)
- Ralloc.c  (arch-dependent register pools)

The FP directory contains FP-specific codegen routines depending on
Thumb/Thumb2/VFP/PortableFP:

- Thumb2VFP.c
- ThumbVFP.c
- ThumbPortableFP.c

Then the hierarchy is formed by stacking these files in the following top-down
order:

1 CodegenCommon.c
2 Thumb[2]/Factory.c
3 CodegenFactory.c
4 Thumb[2]/Gen.c
5 FP stuff
6 Thumb[2]/Ralloc.c
7 CodegenDriver.c