wc: increase I/O size from 16 KiB to 256KiB

wc: increase I/O size from 16 KiB to 256KiB

Similarly to commit v9.4-143-gfcfba90d0,
and enabled for AVX by commit v9.5-25-g0e4450103.

This was seen to improve AVX performance by about 10%
on an AMD 7800X3D (Ryzen 7 (2023)) CPU,
while having neutral AVX performance,
on an Intel i7-5600U (Broadwell-U (2015)) CPU.
With avx not enabled, this gives about a 3% performance boost,
on an Intel i7-5600U.

* src/wc.c: Use the centrally configured optimum buffer size.
* src/wc_avx2.c: Likewise.
* NEWS: Mention the change in performance.