The dependency on sse4.1 crashes on older cpus, use the generic musl
implementation.

musl doesn't define __isnan, so we must mark as a C function.

Not 100% it is right wrt SNaN, but it will fault if incorrect.

This takes the short cut of using sse 4.1 for the implementation,
avoiding a coplicated C implementation.  Will work on Core and higher
processors.