Improve accuracy of SIMD exp for small args

Improve accuracy of SIMD exp for small args

Introduce a separate check for small arguments and
set the result to zero below this cutoff. This change
also moves the exponential tests to ldexp(), which
as a side-effect also makes that function safer by
default, with an optional template parameter to avoid
the checks.

Fixes #2243.

Change-Id: I41bccaeec9921c3aead2cd2caf41cbe2206e0687