Use exp implementation only for SSE & AVX
Use exp implementation only for SSE & AVX

Previously, the exp(Vector<float, Abi>) implementation would match for
any VectorAbi. Thus, you'd get ambiguity for exp(Scalar::float_v),
breaking exp(SimdArray<float, N>) for "odd" N. Now an enable_if resolves
the ambiguity.

Refs: gh-75
Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc