Removed @self and @Trait.
auto merge of #12030 : eddyb/rust/kill-at-self-and-trait, r=cmr
Add basic backtrace functionality

Whenever a failure happens, if a program is run with
`RUST_LOG=std::rt::backtrace` a backtrace will be printed to the task's stderr
handle. Stack traces are uncondtionally printed on double-failure and
rtabort!().

Closes #10128