Initial support for NuttX (#3909)

[no1wudi]

Define the essential types that for NuttX OS.

This is picked from #3909

• 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

[rustbot]

r? @tgross35

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[tgross35]

I got this backport in a group #3968. Usually I do them periodically and catch the stable-nominated issues. All the same, thanks for putting this up!