RFR: 8239927: Product variable PrefetchFieldsAhead is unused and should be removed [v4]

[Bhavana-Kilambi]

> The product variable "PrefetchFieldsAhead" is defined in gc_globals.hpp and set in vm_version_x86.cpp. 
> But as it's not used anywhere, removing this option from the JDK source.

Bhavana-Kilambi has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Merge master
 - 8239927: Product variable PrefetchFieldsAhead is unused and should be removed

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Changes: https://git.openjdk.java.net/jdk/pull/6783/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6783&range=03
  Stats: 13 lines in 3 files changed: 1 ins; 10 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6783.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6783/head:pull/6783

PR: https://git.openjdk.java.net/jdk/pull/6783