target/arm: Introduce ARM_FEATURE_V8_ATOMICS and initial decode

target/arm: Introduce ARM_FEATURE_V8_ATOMICS and initial decode

The insns in the ARMv8.1-Atomics are added to the existing
load/store exclusive and load/store reg opcode spaces.
Rearrange the top-level decoders for these to accomodate.
The Atomics insns themselves still generate Unallocated.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180508151437.4232-8-richard.henderson@linaro.org
[PMM: Drop the ARM_FEATURE_V8_1 feature flag]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>