AArch64: Change arm64 backend to produce A64 code.

The arm backend clone is changed to produce A64 code. At the moment
this backend can only compile simple methods (both leaf and non-leaf).

Most of the work on the assembler (assembler_arm64.cc) has been done.
Some work on the LIR generation layer (functions such as OpRegRegImm
& friends) is still necessary. The register allocator still needs to
be adapted to the A64 instruction set (it is mostly unchanged from
the arm backend). Offsets for helpers in gen_invoke.cc still need to
be changed to work on 64-bit.

Change-Id: I388f99eeb832857981c7d9d5cb5b71af64a4b921