Bump global JTReg memory limit to 768m

This is mostly motivated by JDK-8267241, but has an additional bonus of harmonizing the test configurations across components.

Current JTReg defaults to -Xmx512m, with two exceptions:
 - hotspot does not set default heap at all (AFAICS, relying on MaxRAMPercentage and explicit heap sizes instead);
 - langtools set it to -Xmx768m

I would propose to bump the default limit to 768m, and drop the langtools exception that is now subsumed by new default. The fact that there seem to be no complaints in running heavily-parallel langtools tests with -Xmx768m hopefully tells us that heavily-parallel jdk tests would survive too.