Fix out-of-range integer literals for threefry

Fix out-of-range integer literals for threefry

The 64-bit literals need to have LL suffixes, or
xlc will warn about out-of-range literals and generate
code that fails the unit test. There was also a
single 64-bit literal in the implementation. Other
compilers appear to have handled this correctly,
so it is unlikely to have caused any user bug.

Refs #1725.

Change-Id: I575b6c12ace14973d4fae1edc81836686a2ff9d5