ARM: VIXL32: Add an initial code generator that passes codegen_tests.

This VIXL32-based code generator is not enabled in the optimizing
compiler by default. Changes in codegen_test.cc test it in parallel
with the existing ARM backend.

This patch provides a base for further work, the new backend will not
be enabled in the optimizing compiler until parity is proven with the
current ARM backend and assembler.

Test: gtest-codegen_test on host and target

Change-Id: Id556a975b2645bf1d98ab2984650e8435b2312c2