refactor: deprecate ELAST constants

[dybucc]

Description

This set of constants had already been discussed to cause some issues in #3131. This patch
deprecates such bug-prone and latest-error symbols such that the interface to the libc crate is
not prone to SemVer-breakage as often as the values change upstream.

Note the NetBSD constant was left unmodified as it already had been annotated with a deprecation
attribute.

If you yourself want to help out in the deprecation effort for this type of constants across the
libc crate, maybe try out the libc-constant-deprecator crate. It's been the tool
used to perform the changes that have been submitted in this PR, and it very much welcomes bug
reports.

Sources

• Upstream source code for the FreeBSD ELAST macro declaration, mentioning its use as a "limit"
  value for other errno values.
  https://github.com/freebsd/freebsd-src/blob/bd15d6ef126ee4c0eac931117f6bbbf6f9a3fc72/sys/sys/errno.h#L184
• Upstream source code for the OpenBSD equivalent macro declaration.
  https://github.com/openbsd/src/blob/58ff4898d99200dea776fa757004330318255d45/sys/sys/errno.h#L175
• Upstream source code for the XNU kernel equivalent macro declaration.
  https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/sys/errno.h#L270
• Upstream source code for the DragonFlyBSD equivalent macro declaration.
  https://github.com/DragonFlyBSD/DragonFlyBSD/blob/7b7fb99bddde8ded0bee8f43df4026ca02be0938/sys/sys/errno.h#L206

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]

Some changes occurred in a NetBSD-like module

cc @semarie

Some changes occurred in an OpenBSD module

cc @semarie

[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.