Rollup merge of #36794 - japaric:target-panic, r=alexcrichton

Rollup merge of #36794 - japaric:target-panic, r=alexcrichton

add a panic-strategy field to the target specification

Now a target can define its panic strategy in its specification. If a
user doesn't specify a panic strategy via the command line, i.e. '-C
panic', then the compiler will use the panic strategy defined by the
target specification.

Custom targets can pick their panic strategy via the "panic-strategy"
field of their target specification JSON file. If omitted in the
specification, the strategy defaults to "unwind".

closes #36647

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I checked that compiling an executable for a custom target with "panic-strategy" set to "abort" doesn't need the "eh_personality" lang item and also that standard crates compiled for that custom target didn't contained undefined symbols to _Unwind_Resume. But this needs an actual unit test, any suggestion on how to test this?

Most of the noise in the diff is due to moving `PanicStrategy` from the `rustc` to the `rustc_back` crate.

r? @alexcrichton
cc @phil-opp