Test for curve25519-donna-c64 usability is borken.

In legacy/trac#8753 (closed), weasel discovered that under some circumstances, we're building curve25519-donna-c64 on 32-bit operating systems. It looks like our order-of-operations is busted in the testing code for whether 128-bit int ops work, and we hadn't noticed because our return-status boolean was inverted.