[crac] RFR: Keep _features the same after CRaC restore [v2]

[Jan Kratochvil]

> Extending `_features` may change ABI or existing JIT-compiled code. Unfortunately we cannot use more advanced CPU features after restore, we need to stick to the same features of CPU used to make the checkpoint file.

Jan Kratochvil 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 three additional commits since the last revision:

 - Do not modify global variables in crac_restore()
 - Merge branch 'crac' into crac-keepfeatures
 - Keep _features the same after CRaC restore

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

Changes:
  - all: https://git.openjdk.org/crac/pull/103/files
  - new: https://git.openjdk.org/crac/pull/103/files/93435f79..fda35d13

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=crac&pr=103&range=01
 - incr: https://webrevs.openjdk.org/?repo=crac&pr=103&range=00-01

  Stats: 25432 lines in 1022 files changed: 12003 ins; 6328 del; 7101 mod
  Patch: https://git.openjdk.org/crac/pull/103.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/103/head:pull/103

PR: https://git.openjdk.org/crac/pull/103