macros: define VC_IMPL_POPCNT whenever possible
macros: define VC_IMPL_POPCNT whenever possible

I don't want to have to specify POPCNT - this is a bit special because
it's much less related to the SIMD instruction selection. So unless the
implemention is set to Scalar, if __POPCNT__ is defined define
VC_IMPL_POPCNT.

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

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