RFR: 8074319: barrier_set_cast defined via friend injection

[Kim Barrett]

Please review this small change to the recently added barrier_set_cast
(see https://bugs.openjdk.java.net/browse/JDK-8069016). The way that
new function is being defined is unintentionally not standards
conforming and should not compile, but all of our present official
test platforms accept it anyway. The code is presently only known to
fail with the more recent Xcode6.

I will need a sponsor for this change.

CR:
https://bugs.openjdk.java.net/browse/JDK-8074319

Webrev:
http://cr.openjdk.java.net/~kbarrett/8074319/webrev.00/