VFPU: Ensure that sin(4*x) returns 0.0 (and cos 1) for all x. Fixes #2921

This also makes it easier to replace our VFPU sin/cos implementations with some approximation later.

Note that our sinf/cosf function replacements should not call these, they are expected to behave like libc sinf/cosf, I would think.

Fixes #2921