XCore target has -fno-use-cxa-atexit as default.
XCore target Type defines.

Change SizeType, PtrDiffType, IntPtrType, WCharType, WIntType
to follow the XMOS llvm-gcc front end's settings.
XCore target requires preferred alignment.

The xcore llvm backend does not handle 8 byte alignment viz:
  "%BadAlignment = alloca i64, align 8"
So getPreferredTypeAlign() must never overalign.