Fix coshf and sinhf early overflow

Change test to reveal early overflow in sinhf/coshf,
and fix it.

A better approach would delay the overflow in expk2f
rather than using a different formula, but would it
preserve accuracy?

Decide whether to inline special case
depending on performance.
Could increase register pressure but should also
be able to re-use expk2f coefficients.

Current approach shows performance improvement
despite additional logic.