Separate L4Re from Linux code, add aarch64 and enable tests

[farao]

The L4Re code was previously attached to the Linux code which was not correct in many ways. This commit separates the L4Re code and enables the libc-tests and includes the fixes for the failing tests. Aarch64 is added as a second supported architecture (more to come).

Sources

L4Re-adapted version of uclibc: https://github.com/kernkonzept/l4re-core/tree/master/uclibc/lib.

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 the Android module

cc @maurer

[farao]

Hi @tgross35, this is the refactoring of the recent L4Re PR: #4383 (which is kept around just in case).

I think the failure of the freebsd nightly checks is not my fault. The rest succeeds now.

I did most of what you requested in your comment on the old PR, I just kept the linux/mod.rs file around since there is a massive part of code that is linux-only and is not supported by L4Re and it did not seem to make sense to put it all in shared.rs.

Also, I put some more code that's shared from the sub modules (emscripten, android, linux, l4re) up in linux_like/mod.rs. In theory, I think there's potential to do that with more code, that's just the one that I would have put in shared.rs but realized that it could go into linux_like/mod.rs instead.