libc: transition differing bit-width time and offset variants and deprecate bug-prone constants

This proposal focuses mainly on two points of the `libc` issue tracking the `1.0` release. The first of these addresses the presence of 64-bit and 32-bit wide variants of time- and file offset-related functionality for which it has been stated that only a single, 64-bit type should exist. This should also solve the concern about which of the bit-width variants should `libc` offer, as briefly discussed in the latter comments of https://github.com/rust-lang/libc/pull/5010#issuecomment-4060131422. This proposal also attempts to deprecate a set of constants which, as described in https://github.com/rust-lang/libc/issues/3131, are prone to bugs, but most importantly, require a large amount of manual effort to deprecate in whole. The goal here would be to come up with a solution that, while still manual, would allow bulk deprecation of these symbols throughout the codebase.