RFR+RFA [11u] (S) 8219260: Default number of test jobs needs to be consistently calculated

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8219260

Fix:
  http://cr.openjdk.java.net/~shade/8219260/webrev.01/

This is the 11u-specific fix, the partial backport of the large change in 12u. See the full story in
the bug. This fix is needed to pass tier1 on decently large machine with not much memory. For
example, this allows my 32-thread Threadripper with 64 GB of memory to pass tier1 test suite with
elevated heap size due to JDK-8219251. The alternative is to guess and put TEST_JOBS param to the
test line.

Testing: Linux x86_64 fastdebug, test/langtools, tier1 on a few machines

Thanks,
-Aleksey