arc: use different load instruction

If the offset within the thread struct to the
ARC arch-specific 'relinquish_cause' member is too
large, ld_s instructions referencing it will not
compile. This happens easily if CONFIG_THREAD_NAME
reserves a name buffer within the thread struct, since
all the arch-specific members come last.

Use the regular 'ld' instruction instead.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>