split asm files by architecture (x86 -> i386,x86_64 ; arm -> arm,arm64)
pull in initial version of compile-arm64.S, to fix arm64 build (untested)

Initial implementation courtesy Joel Dice (@dicej)
fix compile-arm64.S when using the apple toolchain
Merge pull request #379 from joshuawarner32/master

ASM refactoring
work in progress towards 64-bit ARM JIT support

This won't build, it's just a snapshot of what I have so far.

Conflicts:
	include/avian/codegen/architecture.h
	include/avian/codegen/registers.h
	src/codegen/compiler.cpp
	src/codegen/compiler/event.cpp
	src/codegen/compiler/site.cpp
	src/codegen/compiler/site.h
	src/codegen/registers.cpp
	src/codegen/target/arm/assembler.cpp
	src/codegen/target/arm/registers.h
snapshot of ARM64 instruction encoding work

Still not building, but more progress.
fix some merge-introduced problems