MFC: r214526

MFC: r214526

Partially revert r203829 (MFC'ed to stable/7 in r205920); as it turns out
what the PowerPC OFW loader did was incorrect as further down the road
cons_probe() calls malloc() so the former can't be called before init_heap()
has succeed. Instead just exit to the firmware in case init_heap() fails
like OF_init() does when hitting a problem as we're then likely running in
a very broken environment where hardly anything can be trusted to work.

git-svn-id: svn://svn.freebsd.org/base/stable/8@214690 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f