Auto merge of #37833 - sfackler:process-abort, r=alexcrichton
Auto merge of #37833 - sfackler:process-abort, r=alexcrichton

Add std::process::abort

This calls libc abort on Unix and fastfail on Windows, first running
cleanups to do things like flush stdout buffers. This matches with libc
abort's behavior, which flushes open files.

r? @alexcrichton

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