Support for Wasm32 Linux Target

[arjunr2]

• Edit corresponding file(s) under libc-test/semver when you add/remove item(s), e.g. edit linux.txt if you add an item to src/unix/linux_like/linux/mod.rs
• Your PR doesn't contain any private or unstable values like *LAST or *MAX (see #3131)
• If your PR has a breaking change, please clarify it
• If your PR increments version number, it must NOT contain any other changes (otherwise a release could be delayed)
• Make sure ci/style.sh passes
• cd libc-test && cargo test
  • (this might fail on your env due to environment difference between your env and CI. Ignore failures if you are not sure)

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[arjunr2]

Notes about the new target wasm32-linux-musl, added in this rustc fork to implement a Wasm Linux Interface

• Patching of cc, ctest, and libtest are still under progress for this target but should be ready soon. It doesn't impact any other existing linux or wasm target
• Without libtest, the harness=true tests don't run, but semver passes