RFR: 8272807: Permit use of memory concurrent with pretouch

[Kim Barrett]

[Trying again, after a couple previous proposals.]

Please review this change to os::pretouch_memory to permit use of the memory
being pretouched concurrently with the pretouch operation.  This will allow
future changes that can make use of such overlap.

Testing:
mach5 tier1-3 with -XX:+AlwaysPreTouch.

Verified no performance impact by comparing startup times with Epsilon GC and
-XX:+AlwaysPretouch on a large enough heap that the pretouch takes a
significant amount of time.  Tested on both linux-x64 and linux-aarch64.

None of these test concurrent use; that will wait for future changes that use
the new feature.

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Commit messages:
 - pretouch allowing concurrent access

Changes: https://git.openjdk.java.net/jdk/pull/7343/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7343&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272807
  Stats: 27 lines in 2 files changed: 17 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7343.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7343/head:pull/7343

PR: https://git.openjdk.java.net/jdk/pull/7343