wc: improve aarch64 Neon optimization for 'wc -l'

    $ yes abcdefghijklmnopqrstuvwxyz | head -n 200000000 > input
    $ time ./src/wc-prev -l input
    200000000 input

    real	0m1.240s
    user	0m0.456s
    sys	0m0.784s
    $ time ./src/wc -l input
    200000000 input

    real	0m0.936s
    user	0m0.141s
    sys	0m0.795s

* configure.ac: Use unsigned char for the buffer to avoid potential
compiler warnings. Check for the functions being used in src/wc_neon.c
after this patch.
* src/wc_neon.c (wc_lines_neon): Use vreinterpretq_s8_u8 to convert 0xff
into -1 instead of bitwise AND instructions into convert it into 1.
Perform the pairwise addition and lane extraction once every 8192 bytes
instead of once every 64 bytes.
Thanks to Lasse Collin for spotting this and reviewing a draft of this
patch.

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