XCore target: Enable frames larger than 65535 to be lowered
XCore target: Fix eliminateFrameIndex() to handle large frames

Large frame offsets are loaded from the ConstantPool.
Where possible, offsets are encoded using the smaller MKMSK instruction.
Large frame offsets can only be used when there is a frame-pointer.
XCore target: extend tests in preparation
XCore target: Add large code model

When using large code model:
Global objects larger than 'CodeModelLargeSize' bytes are placed in sections named with a trailing ".large"
The folded global address of such objects are lowered into the const pool.

During inspection it was noted that LowerConstantPool() was using a default offset of zero.
A fix was made, but due to only offsets of zero being generated, testing only verifies the change is not detrimental.

Correct the flags emitted for explicitly specified sections.

We assume the size of the object queried by getSectionForConstant() is never greater than CodeModelLargeSize.
To handle greater than CodeModelLargeSize, changes to AsmPrinter would be required.
XCore target: fix large code model 'select' indirect address handling.