Make it harder for compiler to optimize away SIMD CMake tests

On at least one old version of Linux, a new compiler in
combination with an old assembler led to the compiler
understanding the SIMD code but optimizing it away, which made
the test pass even though the assembler could not handle it.
This changes the return value of the CMake tests to be based on
the SIMD operations, which should make them a lot more
difficult to optimize away.

Fixes #1493.

Change-Id: I3e021c3c718cf54afaadf131c5fa911b3933f61e