Remove MACHINE_ARCH=amd64 and legacy make instructions from makefiles

Apparently the x86_64 platform used to be referred to as "amd64" so some
makefile code was added to help with the transition.  It probably should
have been removed when bringing bmake in.

With the commit, the makefiles expect bmake and will break if an older
make is used (e.g. from DragonFly 3.2).  That means an upgrade to
DragonFly 3.6 will have to be upgraded to DragonFly 3.4 first.  There
may be other reasons to do this as well, besides just bmake.

The recent symbol versioning makefile for libraries requires bmake, so
legacy make can't build world anymore in any case.