dwdebug/ now supports symbol location resolution in frames on a call
stack.  Only the necessary junk for location resolution is in dwdebug;
all the rest of the unwinding logic is in target/.  dwdebug/ only
knows about a "current frame" and the ability to use symbol info from
other frames.  It assumes the interface in struct location_ops.  I added
CFI register rule evalution, improved CFI parsing, etc.

(I had first written it with more stack context awareness in dwdebug, but
this got icky, so I rewrote it so dwdebug did not need to know.)

So, of course, all the target/ location resolution/loading APIs change,
because the core resolution/loading functions must be able to load not
only in frame 0, but in frame N.

Anyway, the result is an unwinding API that works (see tools/backtrace.c).

This also enables function argument value loading for C++ programs
compiled in recent gccs (which try to use the CFI cfa value as the
frame_base pseudo register, in my limited testing).  C++ support was
therefore of limited utility until this commit.