Add support for alternative QNX Neutrino network stack io-sock

[flba-eb]

Description

QNX 7.1 supports two network stacks:

1. io-pkt, which is default
2. io-sock, which is optional and should be default in QNX 8.0

As one can see in the io-sock migration notes, this changes the libc API in a way similar to e.g. linux-gnu vs. linux-musl.

This change uses a new target which has a different value for target_env, so that e.g. libc can distinguish between both APIs. New target pull request for rustc: rust-lang/rust#133631

CC: @jonathanpallant @japaric @gh-tr @AkhilTThomas

Sources

• Relevant tests in libc-test/semver have been updated
  ⚠️ QNX is unfortunately not ABI compatible between versions, hence semver checks are not possible.
• 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

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[bors]

☔ The latest upstream changes (presumably #4198) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #4202) made this pull request unmergeable. Please resolve the merge conflicts.