RFR (S): 8067469: G1 ignores AlwaysPreTouch

[Thomas Schatzl]

Hi all,

  can I have reviews for the following small change?

It adds pre-touching of allocated memory in G1, which has been forgotten
to be implemented when changing this code in JDK-8038423.

This change adds this and refactors the pre-touch code to a single
method in the os class.

CC'ed runtime since it changes code in os.hpp/cpp.

CR:
https://bugs.openjdk.java.net/browse/JDK-8067469
Webrev:
http://cr.openjdk.java.net/~tschatzl/8067469/webrev/

Testing:
jprt, manual testing with and without AlwaysPreTouch enabled

Thanks,
  Thomas