Add an unwind log Printf to note when an eh_frame section is

loaded/parsed.  Should add timers to this eventually.

Delay getting a full UnwindPlan if it's possible to unwind with
just a fast UnwindPlan.  This keeps us from reading the eh_frame
section unless we hit something built -fomit-frame pointer or we
hit a frame with no symbol (read: no start address) available.

It doesn't look like it is correctly falling back to using the
full UnwindPlan to provide additional registers that the fast
UnwindPlan doesn't supply; e.g. go to the middle of a stack and
ask for r12 and it will show you the value of r12 in frame 0.
That's a bug for tomorrow.

llvm-svn: 117361