8u backport RFR: 6515172 and 8148987 (Linux availableProcessors fixes)

[Andreas Eriksson]

Hi,

Please review the backport of these two fixes:
6515172: Runtime.availableProcessors() ignores Linux taskset command
8148987: [Linux] Allow building on older systems without CPU_ALLOC support

Bugs:
https://bugs.openjdk.java.net/browse/JDK-6515172
https://bugs.openjdk.java.net/browse/JDK-8148987

Webrevs:
http://cr.openjdk.java.net/~aeriksso/6515172/webrev.00/
http://cr.openjdk.java.net/~aeriksso/8148987/webrev.00/

Changes to jdk9:
No unified logging, so a new Linux-only diagnostic flag, 
PrintActiveCpus, added instead.
A few changes to the test were needed because of testlibrary differences.

Thanks,
Andreas