RFR: JDK-8196985: Disable new warnings from GCC 7.3 in jdk libraries

[Erik Joelsson]

When building the jdk with GCC 7.3, a bunch of new warnings are 
triggered. These include:

* implicit-fallthrough
* shift-negative-value
* misleading-indentation
* maybe-uninitialized

I won't attempt to fix any of these at this point, but rather just 
disable the warnings. I have created followup issues (or added to 
existing ones where applicable) for each library/executable where 
warnings need to be evaluated.

Bug: https://bugs.openjdk.java.net/browse/JDK-8196985

Webrev: http://cr.openjdk.java.net/~erikj/8196985/webrev.01/

/Erik