Disable `libregex` for QNX 7.0

[nyurik]

libregex did not exist until QNX 7.1. Before that, the regex functions were part of the libc.

At the moment, this is a NOOP because nto70 only supports no_std.

I am working on backporting std support to 7.0, and hit this bug. See https://github.com/rust-lang/rust/pull/127897

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) some time within the next two weeks.

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[joshtriplett]

r=me once CI passes.

[nyurik]

@joshtriplett thx for the review and comment fix. CI seems to be failing on something unrelated

[nyurik]

@joshtriplett let me know if there is anything else needed here - the CI seem to be failing for unrelated reasons. This PR is needed to merge QNX 7.0 support into the compiler. Thx!

[nyurik]

r? @joshtriplett