arc: Preserve STATUS32 flags while setting AE flag

KFLAG instruction might affect multiple flags in STATUS32 register
and so when we need just AE-bit to be set we need first read current
state of STATUS32, then change our bit and set STATUS32 again.

Otherwise critical flags including stack checking, unaligned access etc
will be dropped for good.

Signed-off-by: Watson Zeng <zhiwei@synopsys.com>