RFR(XS): 8193509: Test dynamic path to retrieve active processor count.

[Lindenmaier, Goetz]

Hi,

active_processor_count() in os_linux.cpp has two paths, one is only
excercised if the VM runs on a machine with more than 1024 processors.
Flag UseCpuAllocPath allows to force this path. Add a test with
this flag to excercise this code.

Please review this change. I please need a sponsor. (Do I need one with the new repo setup?)
http://cr.openjdk.java.net/~goetz/wr17/8193509-activeProcTest/webrev.01/

Best regards,
  Goetz.