arc: fix relocation for big-endian target

In case of little-endian ARC700 instructions (which may include target
address) are encoded as middle-endian. That's why it's required to swap
bytes after read and ten right before write back.

But in case of big-endian ARC700 instructions are encoded as a plain
big-endian. Thus no need for byte swapping.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Francois Bedard <fbedard@synopsys.com>
Cc: Tom Rini <trini@ti.com>
cc: Noam Camus <noamc@ezchip.com>