wc: add AVX512 function for line counting

wc: add AVX512 function for line counting

* configure.ac: Add detection of AVX512 intrinsics for wc.
* src/local.mk: Build AVX512 wc libraries.
* src/wc.c: Add runtime detection of AVX512 intrinsics and call
appropriate function when detected.
* src/wc.h (wc_lines_avx512): Declare function.
* tests/wc/wc-cpu.sh: Add a test that disables AVX512 intrinsics.
* src/wc_avx512.c: New file containing the wc -l implementation using
AVX512. The logic and code is reused from the AVX2 implementation with
slight adaptations. Replaced __builtin_popcount by __builtin_popcountll
and the combination of _mm256_cmpeq_epi8 and _mm256_movemask_epi8 by a
single call to _mm512_cmpeq_epi8_mask.
* NEWS: Mention the improvement.