Switch back to backtrace().

The reason is that __builtin_frame_address(n) for n > 0 seems to just
crash on most platforms when -fomit-stack-pointer is specified, which
seems to be the default for many platforms on -O2. The documentation
says that __builtin_frame_address() should return NULL in case it can't
get the frame, but it seems to crash instead.

Therefore, this commit reverts to using backtrace() from execinfo.h, if
available. However, as __builtin_frame_address() seems to always work on
PPC (even with -fomit-frame-pointer) and seems to be the only way to get
a backtrace on the Wii, this is still used if backtrace() is unavailable
and __ppc__ defined.