Add SECBIT_ constants from securebits.h

[chriswailes]

Description

This PR adds the SECBIT_ constants for use with the PR_GET_SECUREBITS and PR_SET_SECUREBITS prctl operations.

Sources

linux/securebits.h

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

@rustbot label +stable-nominated

[chriswailes]

I don't believe that the FreeBSD failures are due to the changes in this pull request.

[tgross35]

The values look good to me, but in the interest of matching the source code would you mind adding a const fn issecure_mask to do the shifts? Something like

const SECURE_NOROOT: c_int = 0;
pub const SECBIT_NOROOT: c_int = issecure_mask(SECURE_NOROOT);

// ...

const fn issecure_mask(x: c_int) -> c_int {
    1 << x
}