[Target] Make key=arm_cpu --> key=arm_cpu,cpu on AArch64

[AndrewZhaoLuo]

The key in a target is used in dispatching to different strategies.

In most of the code base arm_cpu implies also having cpu. E.g. in python/tvm/target/target.py:

def arm_cpu(...):
    opts = ["-keys=arm_cpu,cpu", "-device=arm_cpu"] + pre_defined_opt
    opts = _merge_opts(opts, options)
    return Target(" ".join(["llvm"] + opts))

In src/target/parsers/mprofile.cc:

static Array<String> MergeKeys(Optional<Array<String>> existing_keys) {
  const Array<String> kExtraKeys = {"arm_cpu", "cpu"};

  if (!existing_keys) {
    return kExtraKeys;
  }

  Array<String> keys = existing_keys.value();
  for (String key : kExtraKeys) {
    if (std::find(keys.begin(), keys.end(), key) == keys.end()) {
      keys.push_back(key);
    }
  }
  return keys;
}

However A-series targets did not have both arm_cpu and cpu keys. Rather they only had arm_cpu keys. This PR makes it so it includes both, matching everywhere else in codebase.

This also may lead to increased performance speedups as previously A-series targets would dispatch to generic strategy instead of cpu optimized strategy. With this, they should properly dispatch to arm_cpu strategy, and fallback to cpu strategy.

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[Mousius]

Argh, this was an unintended side effect of #12474, thanks for fixing this @AndrewZhaoLuo!