binutils: Make manpage handling generic to ease maintenance

binutils: Make manpage handling generic to ease maintenance

DragonFly has two sets of binutils with one designated as primary and the
other is the backup.  In the recent past, every time these designation
were switched or a new binutils was brought in, around 30 makefiles had
to be manually and tediously updated in at least 3 places per file.

This commit implements common variables that will make future primary
designation changes not require changes to individual makefiles, but
rather limit the update to a couple of commons ones.