Add symbolize support for qnx-nto

[Veykril]

This enables symbol resolution on the QNX neutrino operating system.

These cfgs require the following symbols/definitions from libc which exist for the target:

• mmap
• munmap
  • MAP_FAILED
  • PROT_READ
  • MAP_PRIVATE
• dl_iterate_phdr
  • dl_phdr_info
• size_t
• c_void
• c_int

We are currently doing a port of this target for ferrocene and since backtrace-rs is a submodule in rust-lang/rust, having this addition upstreamed would save us from manually maintaining a fork for these two changes.

[gh-tr]

There is a more complete pull request here: #507

[Veykril]

Since https://github.com/rust-lang/backtrace-rs/pull/507 adds the full support for QNX Neutrino I'll close this in favor of that PR