man: Build manpages for all architectures

Building and installing architecture-specific man pages only raises a number of
problems:

• The https://www.freebsd.org/cgi/man.cgi is incomplete. As an example, it does not show results for pae(4). The reason for this is that the cgi interface runs on FreeBSD amd64.
• In FreeBSD amd64 some manual pages have broken X-refs. See hptrr(4) for an example.
• Also, we have broken links in our Release Notes. This is a consequence of the first point. See https://www.freebsd.org/releases/13.0R/hardware/#proc-i386.

Make MAN_ARCH default to 'all' so we build all the man pages for all the
architectures. The difference in disk space is negligible. Also link
architecture-specific man pages to their own section while keeping their own
namespace.

PR: 212290
Reported by: mj@bsdops.com
Approved by: ceri@, wosch@
MFC after: 4 weeks