core: Rewrite last_os_error in Rust for unix and provide access to errno (unix) and GetLastError (windows).
core: Rewrite last_os_error in Rust for windows as well.
rt: remove last_os_error and adjust tests.
Long lines.
rt: remove last_os_error from rustrt.def.in
auto merge of #4900 : luqmana/rust/core_os_errors, r=graydon

Rewrote the last pull request (#4859) to not require on llvm for core.

Also fixes #2269.