RFR: 8148987: [Linux] Allow building on older systems without CPU_ALLOC support

[David Holmes]

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

webrev: http://cr.openjdk.java.net/~dholmes/8148987/webrev/

This is a tentative RFR as I'm not yet convinced we should support 
building on these older platforms in JDK 9. Opinions welcome on that point.

The older systems do not have dynamic cpu set support (CPU_ALLOC) nor do 
they have the CPU_COUNT utility macro.

Thanks,
David