Document how to use gdb on rustc itself

[jyn514]

This requires me to first figure out how to do it; I know how for standalone files but I tried and failed to do this when compiling the standard library.

[camelid]

cc https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/how.20to.20run.20gdb.20on.20rustc.3F

[igaray]

Also see #443 if you haven't, possible duplicate issue.

[jyn514]

Yup, thanks.