Add a definition of max_align_t to nto

[darkwisebear]

The definition of this type is a bit tricky for nto for multiple reasons:

• The C definition is different from the C++ definition in some versions of the QNX SDK.
• It uses long double, which does not exist inside libc.
• The definition on C uses alignment modifiers per field, which aren't supported in Rust. However, since they just reuse the field type, they're actually redundant so that we can safely skip them.

Description

The change adds a definition for max_align_t for QNX.

Sources

Source of the QNX SDP is closed source and can only be accessed by owning a developer license.

Docs available at https://www.qnx.com/developers/docs/8.0/com.qnx.doc.neutrino.lib_ref/topic/m/max_align_t.html.

Checklist

• Relevant tests in libc-test/semver have been updated (No updates needed as max_align_t isn't part of unix.txt)
• 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 (Test cannot be run for the target also on HEAD; code that uses max_align_t compiles and runs fine, though. Test for host platform runs fine, no surprises here.)

@rustbot label +stable-nominated

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[JohnTitor]

cc target maintainers @flba-eb @gh-tr @jonathanpallant @japaric to verify (as I cannot do on my end)