build: fix man page build failure with some permissions setups

Use the more portable 'chmod a-w', instead of the 'chmod -w' form.
The latter is not always supported.  Also its operation is
dependent on umask controlling the permissions bits for new files,
which is not the case in the presence of POSIX default ACLs for e.g.
In that case, chmod may print a warning like the following, and
exit with failure status:

chmod: man/hostid.1-t: new permissions are r--rw-r--, not r--r--r--

* man/local.mk: s/-w/a-w/

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