Benchmark new Graviton2 ARM EC2 instances [LUCENE-9086]

At [AWS re:Invent 2019](https://reinvent.awsevents.com/) last week, AWS announced new [EC2 instances based on the Graviton2 ARM processor](https://aws.amazon.com/blogs/aws/coming-soon-graviton2-powered-general-purpose-compute-optimized-memory-optimized-ec2-instances) which apparently can be much faster than the original A1 instances, at least according to internal benchmarks.

I've been running Lucene's benchmarks (`wikimediumall`, indexing 33.3 M docs and running a diverse and repeatable set of search tasks) on these instances, comparing a `c4.8xlarge` (x86-64) instance against the new `m6g.8xlarge` (ARM), and I'll summarize the results here.

Net/net ARM seems to be faster at raw indexing than x86-64, even though `m6g.8xlarge` has only 32 cores versus 36 cores, but a bit slower at merging, while searching seems to be faster for some queries and slower for others.  I'll try to get the full results posted soon.

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Migrated from [LUCENE-9086](https://issues.apache.org/jira/browse/LUCENE-9086) by Michael McCandless (@mikemccand), 1 vote, updated Dec 11 2019