Use db_printsym() instead of our own in backtraces

Summary:
Currently if there are problems finding a symbol, backtrace ends up printing
something like:

0xdeadbeef: at +0x12345

Which is pretty useless. This on its own should be fixed (retrieving symbols),
but aside from that, using db_printsym() is a better solution anyway. If it
can't find a valid symbol it prints the actual address, and it has the added
benefit that if it can find the symbol, it might be able to print the file and
line as well.

Test Plan: Tested on my G4 PowerBook

Reviewers: PowerPC, nwhitehorn

Reviewed By: nwhitehorn

Differential Revision: https://reviews.freebsd.org/D1173

MFC after: 3 weeks