Auto merge of rust-lang#148020 - bjorn3:oom_backtrace, r=Mark-Simulacrum

Show backtrace on allocation failures when possible

And if an allocation while printing the backtrace fails, don't try to print another backtrace as that will never succeed.

Split out of rust-lang#147725 to allow landing this independently of a decision whether or not to remove `-Zoom=panic`.

Author: bors