Fixed several libmvec bugs found during testing on KNL hardware.

AVX512 IFUNC implementations, implementations of wrappers to
AVX2 versions and KNL expf implementation fixed.

    * sysdeps/x86_64/fpu/multiarch/svml_d_cos8_core.S: Fixed AVX512 IFUNC.
    * sysdeps/x86_64/fpu/multiarch/svml_d_exp8_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_d_log8_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_d_pow8_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_d_sin8_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_d_sincos8_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_s_cosf16_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_s_expf16_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_s_logf16_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_s_powf16_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_s_sincosf16_core.S: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_s_sinf16_core.S: Likewise.
    * sysdeps/x86_64/fpu/svml_d_wrapper_impl.h: Fixed wrappers to AVX2.
    * sysdeps/x86_64/fpu/svml_s_wrapper_impl.h: Likewise.
    * sysdeps/x86_64/fpu/multiarch/svml_s_expf16_core_avx512.S: Fixed KNL
    implementation.