[ruby/prism] Use a more efficient StringIO on TruffleRuby

* The stdlib StringIO is synchronized and this occurs a high overhead.
* This is about twice as fast on TruffleRuby but surprisingly it is slower on JRuby.
  I am not sure why but probably @ivar access and integer arithmetic
  is much slower than Java field access/arithmetic on JRuby.
* On CRuby interpreter it is slower, which is expected as the GVL already protects StringIO.
* So we enable this only on TruffleRuby to not slow down other Rubies.
* PRISM_FFI_BACKEND=true ruby -v -Ilib -rprism -rbenchmark -e '300.times { p Benchmark.realtime { Dir.glob("lib/**/*.rb") { |f| Prism.parse_file(f) } } }'
  ruby 3.3.0:         0.215 => 0.251 (cext: 0.062)
  ruby 3.3.0 YJIT:    0.118 => 0.113 (cext: 0.053)
  truffleruby JVM:    0.101 => 0.054
  jruby 9.4.6.0:      0.162 => 0.219
  jruby 9.4.6.0 indy: 0.078 => 0.086
* For the record here are the numbers for using the String directly, without a StringIO-like object:
  ruby 3.3.0:         0.215 => 0.234 (cext: 0.062)
  ruby 3.3.0 YJIT:    0.118 => 0.111 (cext: 0.053)
  truffleruby native: 0.101 => 0.053
  jruby 9.4.6.0:      0.162 => 0.195
  jruby 9.4.6.0 indy: 0.078 => 0.082
  As we can see, that extra object adds a non-trivial overhead on CRuby interpreter and JRuby.
  But we need to make it possible to use StringIO and SimpleStringIO interchangeably.

https://github.com/ruby/prism/commit/938677cbd2

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