This is the first part towards a better framework in the build for handling compiler warnings. The basic idea is that we should have consistent way for all compilers to:
1) enable all (relevant) warnings
2) disable individual warnings, on a global scale (if 1 enables too much)

In particular, this means unifying the basic set of enabled warnings between hotspot and the JDK native libraries.

From that starting point, warnings can be disabled on a per-library fashion, if it does not make sense for that specific library. (Like how it's done today, but more consistently performed.)

This first part will just put most of the infrastructure in place, and strives to keep exactly the same warnings enabled or disabled for all code.