Vector pow for x86_64 and tests.

Vector pow for x86_64 and tests.

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

  * bits/libm-simd-decl-stubs.h: Added stubs for pow.
    * math/bits/mathcalls.h: Added pow declaration with __MATHCALL_VEC.
    * sysdeps/unix/sysv/linux/x86_64/libmvec.abilist: New versions added.
    * sysdeps/x86/fpu/bits/math-vector.h: Added SIMD declaration and asm
    redirections for pow.
    * 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_d_wrapper_impl.h: Added 2 argument wrappers.
    * sysdeps/x86_64/fpu/multiarch/svml_d_pow2_core.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_d_pow2_core_sse4.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_d_pow4_core.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_d_pow4_core_avx2.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_d_pow8_core.S: New file.
    * sysdeps/x86_64/fpu/multiarch/svml_d_pow8_core_avx512.S: New file.
    * sysdeps/x86_64/fpu/svml_d_pow2_core.S: New file.
    * sysdeps/x86_64/fpu/svml_d_pow4_core.S: New file.
    * sysdeps/x86_64/fpu/svml_d_pow4_core_avx.S: New file.
    * sysdeps/x86_64/fpu/svml_d_pow8_core.S: New file.
    * sysdeps/x86_64/fpu/svml_d_pow_data.S: New file.
    * sysdeps/x86_64/fpu/svml_d_pow_data.h: New file.
    * sysdeps/x86_64/fpu/test-double-vlen2-wrappers.c: Added vector pow test.
    * sysdeps/x86_64/fpu/test-double-vlen2.c: Likewise.
    * sysdeps/x86_64/fpu/test-double-vlen4-avx2-wrappers.c: Likewise.
    * sysdeps/x86_64/fpu/test-double-vlen4-avx2.c: Likewise.
    * sysdeps/x86_64/fpu/test-double-vlen4-wrappers.c: Likewise.
    * sysdeps/x86_64/fpu/test-double-vlen4.c: Likewise.
    * sysdeps/x86_64/fpu/test-double-vlen8-wrappers.c: Likewise.
    * sysdeps/x86_64/fpu/test-double-vlen8.c: Likewise.
    * NEWS: Mention addition of x86_64 vector pow.