CRIS atomics revisited 0/4: summary

[Hans-Peter Nilsson <hans-peter dot nilsson at axis dot com>]

These were spotted while debugging usage of atomics within
glibc.  The kind of changes are microoptimizations,
nanooptimizations, a buglet and a major issue.  Micro: the
load-store-conditional sequence for compare-and-swap I
originally committed was an earlier version improved later.
Nanooptimizations: choosing better-fitting operands for the
atomic operator insn.  Buglet: a post-increment could have
sneaked into the (non-atomic) arithmetic operator operand;
better make it nonmemory_operand altogether.  I also threw in
use of the now generic need_atomic_barrier_p, let's call that a
microoptimization.  The major issue, giving up on alignment of
atomic data by default, is last.  Tested together and some
separately, no regressions for cris-elf nor crisv32-elf.
Committed separately.

brgds, H-P