[12] RFR: JDK-8215435: Warn on usage of trampolines with gcc

[Kevin Rushforth]

Please review the following fix to add the  "-Werror=trampolines" to gcc:

https://bugs.openjdk.java.net/browse/JDK-8215435
https://github.com/javafxports/openjdk-jfx/pull/345

-- Kevin