With --enable-bfd-backtraces, use binutils libbfd to resolve backtraces

With --enable-bfd-backtraces, use binutils libbfd to resolve backtraces

The invocation of addr2line to resolve backtrace source locations
is slow and cumbersome. When using libbfd directly, we can eliminate
the overhead of the process invocation. Even better, we can cache
library symbol names, bringing wicked fast lookups. As a neat bonus,
we can resolve static function names.