RFR: 8349713: [leyden] Memory map the cached code file

[Aleksey Shipilev]

It is visible in profiles for lots of applications that reading the SC cache file at startup costs significantly. On JavacBenchApp example, loading ~25M code requires about 30ms. This is ~1 GB/sec, so it is I/O limited.

We should really mmap the SC cache file to alleviate these costs. Let the actual SC readers (separate threads) to eat the cost of reading from the backing file. 

I was not entirely sure COW for file mappings works correctly on Windows, so I excepted that one.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `runtime/cds`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/leyden/pull/34/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=34&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349713
  Stats: 33 lines in 2 files changed: 21 ins; 2 del; 10 mod
  Patch: https://git.openjdk.org/leyden/pull/34.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/34/head:pull/34

PR: https://git.openjdk.org/leyden/pull/34