cmake: disable AVX dependent tests if AVX is broken
cmake: disable AVX dependent tests if AVX is broken

clang 3.6.x sets Vc_AVX_INTRINSICS_BROKEN but still compiles and runs
the AVX ABI tests and the supportfunctions test. That's not useful,
since they're expected to not work correctly anyway. So disable them in
the buildsystem if Vc_AVX_INTRINSICS_BROKEN.

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

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