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

[Tim Bell]

Erik:
> 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/

Looks good.

Tim