Support installing manpages for multiple or alternate architecture(s).

Some section-4 manpages are architecture-specific, and the build process
currently generates only the pages for the MACHINE_CPUARCH being built.
man(1) supports a '-m' option to find manpages belonging to an arbitrary
architecture other than the MACHINE_[CPU]ARCH, but we have no way to
generate and install alternate-arch pages right now.

This change adds a new make.conf variable, MAN_ARCH, which can be a list of
one or more MACHINE_ARCH or MACHINE_CPUARCH values. All arch-specific
manpages that exist for the named arches will be installed. If unset, it
continues the behavior of installing just the MACHINE_CPUARCH being built.

Differential Revision: https://reviews.freebsd.org/D16198