RFR: JDK-8215400: Warn on usage of trampolines with gcc

[Erik Joelsson]

Hello,

This small patch adds -Wtrampolines to the default set of warnings 
applied globally in the OpenJDK build. We do not trigger this warning 
today, but by adding it, combined with -Werror, we make sure we never will.

Bug: https://bugs.openjdk.java.net/browse/JDK-8215400

Webrev: http://cr.openjdk.java.net/~erikj/8215400/webrev.01/

/Erik