Split qnx target based on target env

[AkhilTThomas]

• Created nto7071 and nto80 modules.
• Merged neutrino.rs into parent module.

Description

This PR is a preparation to implement the QNX 8.0 support by splitting the implementation logic into separate modules.
It's similar to the approach used by freebsd to support multiple versions which have a different "C" function / structure / constants.

• No functional changes are added to the targets nto70 , nto71 and nto71_iosock.
• The target nto80 is currently identical to the nto7071 implementation to keep the files diff-able.

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI