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
finish implementing operations64.cpp for ARM64 support

This is totally untested so far.
remove 64-bit code from compile-arm.S since it's moved to compile-arm64.S
various ARM64 JIT bugfixes

Three of the tests now pass.  Yay!
more ARM64 bugfixes, more passing tests
more ARM64 bugfixes
more ARM64 bugfixes
more ARM64 bugfixes
remove redundant class qualifiers from Classes.java
fix offset encoding for strhi instruction
fix ARM64 OffsetPromise::value

I must have done a search-and-replace from 4 to TargetBytesPerWord
earlier, but in this case it should have been the instruction size
(4), not the word size.
format recent changes using clang-format
add todo comment regarding integer division
update CMakeLists.txt to fix CMake build
conditionally compile ARM operations based on TARGET_BYTES_PER_WORD

This fixes the codegen-targets=all build regression.
fix ARM64 bootimage=true build

This fixes a problem with atomically updating JIT-compiled static
calls to AOT-compiled code.  It turns out there was also a problem
with the 32-bit ARM code as well, but we never hit it because it is
extremely unlikely that a code address can be loaded with a single
immediate load instruction on 32-bit ARM since it can only handle
numbers with 8 significant bits.  I've fixed that as well.
fix ARM64 tails=true build
add continuations support for ARM64

Also, replace some preprocessor conditionals with C++ conditionals and
add some todo comments and sample code for future work towards better
ABI compatibility in the JIT compiled code.