Errored: OpenSmalltalk/opensmalltalk-vm#1941 (Cog - 811df75)

CogVM source as per VMMaker.oscog-eem.2705

Cogit:
Add support for a substitute SPReg to cope with the ARMv8's typical 16-byte
stack pointer alignment requirement. Implement NativePopR:& NativePushR:.
Fix compilation warnings for cStack/FramePointerAddress access. Fix
nextProfileTickAddress for 64-bit simulation. Write stackLimitAddress &
vmOwnerLockAddress in the modern style. Move genLoadStackPointers from Cogit
to CogAbstractInstruction where it lives with the other stack load/store
generartors, allowing ARMv8 to easily override to use ldp/stp as desired.

Consequently...
- add NativePopR NativePushR NativeRetN abstract instructions (so far only
NativeRetN needs to be implemented; see ceCaptureCStackPointers)
- add ABICalleeSavedRegisterMask & ABICallerSavedRegisterMask to
CogAbstractRegisters
- Add ABIResultReg and ABIResultRegHigh and nuke the cResultRegister accessor.
- Rename genGetLeafCallStackPointer to genGetLeafCallStackPointers, and have
ceGetSP answer the native stack pointer.

Add support for cache flushing in the dual mapped regime, hence rename
maybeGenerateICacheFlush to maybeGenerateCacheFlush.

Slang:
Sort variables by size so that they occupy a little less space.
reenterInterpreter does *not* need to be exported.
Nuke unused Cogit Lowcode vars when not generating a LowcodeVM.

FFI Plugins:
Let X64 SysV FFI handle passing/returning union

This can be tricky because we can have union in struct, struct in union etc...
So we must correctly peel the union.