RFR: 8349713: [leyden] Memory map the cached code file [v3]

[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`

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Remove flag, allow all platforms, remove C-heap load
 - Merge branch 'premain' into JDK-8349713-mmap-sccache
 - Merge branch 'premain' into JDK-8349713-mmap-sccache
 - Fix

-------------

Changes:
  - all: https://git.openjdk.org/leyden/pull/34/files
  - new: https://git.openjdk.org/leyden/pull/34/files/ce07d999..2f884d8e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=leyden&pr=34&range=02
 - incr: https://webrevs.openjdk.org/?repo=leyden&pr=34&range=01-02

  Stats: 65 lines in 7 files changed: 26 ins; 27 del; 12 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