RFR+RFA [11u] (XS) 8219251: Langtools tests default memory size needs to be 768m

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8219251

Fix:
  http://cr.openjdk.java.net/~shade/8219251/webrev.01/

This is the high priority 11u-specific fix, the partial backport of the large change in 12u. This
makes tier1 testing possible with "make run-test TEST=tier1". Having tests passing cleanly is
necessary for doing anything else in 11u. Read the full story in the bug.

Testing: Linux x86_64 fastdebug, jtreg:test/langtools, tier1

Thanks,
-Aleksey