Rollup merge of #48266 - pietroalbini:report-compiler-flags-on-ice, r=michaelwoerister

Report non-standard compile flags on ICE

Some ICEs (such as the recent #48248) only happens when a non-standard compiler flag is provided to rustc, but users don't always report the used flags. This can slow down reproducing the issue, so this PR shows all the non-standard compiler flags in the ICE error message.

For example, the output of #48248 with this PR is:

```
error: internal compiler error: [...]

thread 'rustc' panicked at [...]
note: Run with `RUST_BACKTRACE=1` for a backtrace.

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: [...]

note: rustc 1.25.0-dev running on x86_64-unknown-linux-gnu

note: compiler flags: -C link-dead-code
```

### Open questions

* At the moment, only `-C` and `-Z` flags are shown by default, and all the ones provided by cargo in a standard build are ignored: I did this to only show the flags that probably caused the ICE, and to remove some noise from the message. This removed flags like `opt-level` and `debuginfo` though, could those be useful for reproducing ICEs?

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