Looks like the implementation was not ported correctly. Some negations
were forgotten in a certain branch. Here is the original code in musl
that has the negations:

https://github.com/bpowers/musl/blob/94cb2ec2a0ffcb47d24dbf7a30e462505396cf54/src/math/sincosf.c#L66-L67

Resolves #228