target-m68k: add cas/cas2 ops
Implement CAS using cmpxchg. Implement CAS2 using helper and either cmpxchg when the 32bit addresses are consecutive, or with parallel_cpus+cpu_loop_exit_atomic() otherwise. Suggested-by: NRichard Henderson <rth@twiddle.net> Signed-off-by: NLaurent Vivier <laurent@vivier.eu> Reviewed-by: NRichard Henderson <rth@twiddle.net>
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