RFR: 8352042: [leyden] Improve parallel recompilation parallelism

[Aleksey Shipilev]

Parallel precompilation ([JDK-8352042](https://bugs.openjdk.org/browse/JDK-8352042)) uses the default compiler thread policy, that targets the normally executing applications. In assembly phase, we can actually use all threads available on the machine to complete it as quickly as possible.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/leyden/pull/53/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=53&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8352042
  Stats: 17 lines in 1 file changed: 17 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/leyden/pull/53.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/53/head:pull/53

PR: https://git.openjdk.org/leyden/pull/53