RFR: 8255231: Avoid upcalls when initializing the statSampler [v2]

[Claes Redestad]

> Current implementation of the statSampler does upcalls to System.getProperty to collect values for a number of properties that are all provided by the VM itself. And since the sampling starts before any user code run then no property can have changed.
> 
> I suggest refactoring the code so that no upcalls are made normally - while asserting this invariant holds using assert-only upcalls. 
> 
> This is a small startup optimization - reducing the startup sequence by approx. 300k instructions and 70k branches in my linux-x64 setup.

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Revert unrelated changes to perfData

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/802/files
  - new: https://git.openjdk.java.net/jdk/pull/802/files/7b922c2a..5daedb01

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=802&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=802&range=00-01

  Stats: 21 lines in 2 files changed: 17 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/802.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/802/head:pull/802

PR: https://git.openjdk.java.net/jdk/pull/802