SI-6594 - Switch to sneakyThrows instead of Unsafe.throwException as per...

... new jsr166y to avoid issues with Android

(was previously SI-6905 but that was a duplicate)

See: https://issues.scala-lang.org/browse/SI-6594

Since code is lifted from the jsr166y repo and there is no test suite for FJ embedded in scala-lib I assume this is how we roll.

This of course also needs forwardporting.