sim: Use host not target byte order for merging and splitting values

sim: Use host not target byte order for merging and splitting values

When using writes to memory through a struct to merge and extract
multi-word value, it is the endianness of the host, not the target
that affects which order the component words need to be written into
the structure.

Of the 5 functions adjusted here 4 of them are unused.  The 5th,
JOINSIDF will soon be used by the or1k target.

For or1k, simulated on x86-64, this change fixes this function so that
the correct bytes are now returned.

sim/common/ChangeLog:

	* cgen-ops.h (SUBWORDXFSI): Compare HOST_BYTE_ORDER not
	CURRENT_TARGET_BYTE_ORDER.
	(SUBWORDTFSI): Likewise.
	(JOINSIDF): Likewise.
	(JOINSIXF): Likewise.
	(JOINSITF): Likewise.