xtensa: make fast_unaligned store restartable

fast_unaligned may encounter DTLB miss or SEGFAULT during the store
emulation. Don't update epc1 and lcount until after the store emulation
is complete, so that the faulting store instruction could be replayed.
Remove duplicate code handling zero overhead loops and calculate new
epc1 and lcount in one place.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>