Update to latest upstream symbolic and implement brekpad-to-native fallback

[gabrielesvelto]

A couple of changes in this PR:

• It updates fix-stacks to use the latest symbolic, this solves an issue within symbolic that caused dump_syms to abort when built with recent versions of rustc
• It adds a fallback so that when using breakpad symbols if we can't find the symbol for a given file we try to fall back to the file's native debug information rather than return the unsymbolicated line

The macOS tests are failing because of a change that was introduced in upstream symbolic: this change modified the code used for parsing DWARF objects to ignore line entries starting at address 0. The reasoning behind the change is that those entries would be illegal per the DWARF spec and thus the debuginfo must be considered buggy. However, as it happens, our small macOS-specific test has just that kind of entries in the debuginfo. I tried recompiling it with more recent tools from Apple but to no avail.

[gabrielesvelto]

Thanks Kris, I'll open a follow up bug to pull the new version into mozilla-central after merging