Fix types of size, num_symbols in fancy_backtrace. Close #779

What it doesn't fix, though is the fact that libiberty isn't shipped
by homebrew's binutils anymore, cf:
https://github.com/Homebrew/homebrew/pull/20760
and https://gcc.gnu.org/onlinedocs/libiberty/Using.html#Using

Apparently we should be shipping our own sources, but seeing our
delicate build process, it seems unrealistic for now.