Bump minimum GCC from 5.x to 6 for JDK

While working on JDK-8252797 it was observed that an interaction exists between specific versions of GCC/gtests and JDK headers

GCC version 5.x has an issue with the way it handles the headers:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14608

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14608#c13):

We have workarounds for that issue, but we could conceivably end up with a similar issue in the future involving a different header interaction. We could avoid such risks, if we bumped up the min GCC version requirement to version 6