RFR: 8349722: [leyden] Option to block until preloading is complete

[Aleksey Shipilev]

During one of the meetings, @iwanowww suggested there is a diagnostic option to block the execution until all preloaded code is finished loading. For the Java application, it would then look as if the entirety of preload code was loaded "instantaneously", which helps to estimate the performance floor for a given workload.

I have found no such option in current Leyden prototype, though, so this RFE should implement one. 

This is indeed helpful, especially combined with #32, which allows to run with only C2.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/leyden/pull/36/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=36&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349722
  Stats: 7 lines in 2 files changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/leyden/pull/36.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/36/head:pull/36

PR: https://git.openjdk.org/leyden/pull/36