Work in VM towards cross-bitness crossgen (#18245)

Work in VM towards cross-bitness crossgen (#18245)

* Replace sizeof expressions with target-specific constants:

* sizeof(ObjHeader) -> OBJHEADER_SIZE
* sizeof(Object) -> OBJECT_SIZE
* ObjSizeOf(Object) -> OBJECT_BASESIZE
* sizeof(ArrayBase) -> ARRAYBASE_SIZE
* ObjSizeOf(ArrayBase) -> ARRAYBASE_BASESIZE

* Remove ObjSizeOf macro

* Use OBJECT_SIZE in ArrayBase::GetBoundsOffset

* Cast ppObj to CORCOMPILE_GCREFMAP_TOKENS* before dereferencing in FakePromote

* LOG2_PTRSIZE should depend on _TARGET_64BIT_