Fix mapping of acquire/release AMOs to fence operations

AMO.aq should be implemented as AMO;FENCE, whereas AMO.rl should be
implemented as FENCE;AMO.  These had been swapped.  This error does
not affect cacheable accesses using the blocking D$, nor does it
affect accesses to the data scratchpad, nor does it affect accesses
to strongly ordered I/O regions (which is the default).

Cacheable accesses using the nonblocking D$ and accesses to weakly
ordered I/O regions may manifest memory-ordering violations.  For
these accesses, the workaround is to use AMO.aqrl whenever AMO.aq
or AMO.rl had been used.