wc: fix reading of /proc files on aarch64

wc: fix reading of /proc files on aarch64

tests/misc/wc-proc.sh fails when the page size is 64K

* src/wc.c (wc): The lseek adjustment should be based on st_blksize,
rather than on the internal buffer size.  This is significant on
aarch64 where st_blksize in /proc is the 64K (the page size) and
thus larger than the internal buffer.
* src/split.c (main): Even though the similar processing is done
on the internal buffer size, that's based on st_blksize and
so fine in this regard.  Add an assert to enforce this.
Avoid this path for the undocumented ---io-blksize option.