rt/core: Remove last_os_error from rust_builtin and instead use StrError from LLVM.
Fix broken tests.
auto merge of #4859 : luqmana/rust/last_error, r=catamorphism

No need for it to be in written anew in rustrt since LLVM already provides a platform independent way of getting the error string. This pull request just makes it available in core::os.

So that's one less helper function in rustrt. (#4812)