Add support for QNX Neutrino to standard library This change: - adds standard library support for QNX Neutrino (7.1). - upgrades `libc` to version `0.2.139` which supports QNX Neutrino `@gh-tr` ⚠️ Backtraces on QNX require https://github.com/rust-lang/backtrace-rs/pull/507 which is not yet merged! (But everything else works without these changes) ⚠️ Tested mainly with a x86_64 virtual machine (see qnx-nto.md) and partially with an aarch64 hardware (some tests fail due to constrained resources).