Reduce dependency requirements to enable running some benchmarks with miniruby

[rwstauner]

• Embed realtime method to remove dependency on benchmark
• Warn and skip max rss if fiddle fails to load
• Warn and skip writing file if json fails to load

Makes the output differ thusly:

$ WARMUP_ITRS=1 MIN_BENCH_ITRS=2 MIN_BENCH_TIME=0 ruby ~shopify/yjit-bench/benchmarks/protoboeuf-encode/benchmark.rb
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [arm64-darwin23]
itr #1: 102ms
itr #2: 98ms
itr #3: 98ms
RSS: 51.3MiB
MAXRSS: 52912.0MiB
Writing file /Users/rwstauner/src/github.com/ruby/ruby/data/results-ruby-3.3.0-2024-07-16-132905.json
Average of last 2, non-warmup iters: 98ms

$ WARMUP_ITRS=1 MIN_BENCH_ITRS=2 MIN_BENCH_TIME=0 ./miniruby -r./rbconfig ~shopify/yjit-bench/benchmarks/protoboeuf-encode/benchmark.rb
ruby 3.4.0dev (2024-07-16T19:50:00Z master c083a3ffcd) [arm64-darwin23]
itr #1: 96ms
itr #2: 91ms
itr #3: 91ms
RSS: 54.2MiB
Failed to get max RSS: cannot load such file -- fiddle
Failed to write JSON file: cannot load such file -- json
Average of last 2, non-warmup iters: 91ms