Vector powf for x86_64 and tests.

Vector powf for x86_64 and tests.

Here is implementation of vectorized powf containing SSE, AVX,
AVX2 and AVX512 versions according to Vector ABI
<https://groups.google.com/forum/#!topic/x86-64-abi/LmppCfN1rZ4>.

    * sysdeps/unix/sysv/linux/x86_64/libmvec.abilist: New symbols added.
    * sysdeps/x86/fpu/bits/math-vector.h: Added SIMD declaration and asm
    redirections for powf.
    * sysdeps/x86_64/fpu/Makefile (libmvec-support): Added new files.
    * sysdeps/x86_64/fpu/Versions: New versions added.
    * sysdeps/x86_64/fpu/libm-test-ulps: Regenerated.
    * sysdeps/x86_64/fpu/multiarch/Makefile (libmvec-sysdep_routines):
    Added build of SSE, AVX2 and AVX512 IFUNC versions.
    * sysdeps/x86_64/fpu/svml_s_wrapper_impl.h: Added 2 argument wrappers.
    * sysdeps/x86_64/fpu/multiarch/svml_s_powf16_core.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_s_powf16_core_avx512.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_s_powf4_core.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_s_powf4_core_sse4.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_s_powf8_core.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_s_powf8_core_avx2.S: New file.
    * sysdeps/x86_64/fpu/svml_s_powf16_core.S: New file.
    * sysdeps/x86_64/fpu/svml_s_powf4_core.S: New file.
    * sysdeps/x86_64/fpu/svml_s_powf8_core.S: New file.
    * sysdeps/x86_64/fpu/svml_s_powf8_core_avx.S: New file.
    * sysdeps/x86_64/fpu/svml_s_powf_data.S: New file.
    * sysdeps/x86_64/fpu/svml_s_powf_data.h: New file.
    * sysdeps/x86_64/fpu/test-float-vlen16-wrappers.c: Vector powf tests.
    * sysdeps/x86_64/fpu/test-float-vlen16.c: Likewise.
    * sysdeps/x86_64/fpu/test-float-vlen4-wrappers.c: Likewise.
    * sysdeps/x86_64/fpu/test-float-vlen4.c: Likewise.
    * sysdeps/x86_64/fpu/test-float-vlen8-avx2-wrappers.c: Likewise.
    * sysdeps/x86_64/fpu/test-float-vlen8-avx2.c: Likewise.
    * sysdeps/x86_64/fpu/test-float-vlen8-wrappers.c: Likewise.
    * sysdeps/x86_64/fpu/test-float-vlen8.c: Likewise.
    * math/test-float-vlen16.h: Fixed 2 argument macro.
    * math/test-float-vlen4.h: Likewise.
    * math/test-float-vlen8.h: Likewise.
    * NEWS: Mention addition of x86_64 vector powf.