Simplify the reading of RUST_LIBC_UNSTABLE_GNU_*_BITS variables

[snogge]

Description

Use (almost) the same code in build.rs and libc-test/build.rs. Make the decision chain clearer.
Have a single default value that can be overriden in various ways, which should make switching the default much easier.

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

[tgross35]

Appreciate the cleanup, this looks better.

Would it make sense to require that OFFSET_BITS is unset if TIME_BITS is set? I think that may be more straightforward for users, so they only ever set one config at a time.

Another thing we talked about is switching to a rust cfg rather than env, so users can pass RUSTFLAGS='--cfg libc_unstable_gnu_time_bits="64"'. The benefit is this only affects the target, not anything built on the host system. The implementation should be the same if you are up for changing it here, just check CARGO_CFG_LIBC_UNSTABLE_GNU_TIME_BITS and CARGO_CFG_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS rather than the RUST_-prefixed env.

[snogge]

I did a rewrite to require at most one variable to be set. The logic was also wrong and did not support the easy switch of default that I wanted.
This version supports just changing defaultbits to 64 to switch to 64 bit time_t. I hope this difference is small enough to handle on master.

The change to rust cfg requires CI-changes as well. I'll do that separately.