Emit `nounwind` LLVM attribute with `-C panic=abort`

[alexcrichton]

Currently whenever you compile with -C panic=abort we will omit invoke instructions and the uwtable LLVM function attribute, but we should also set the nounwind attribute. It looks like on ARM this forces the object file to not reference functions that otherwise a function missing the nounwind attribute will not have.

This should help out remove some weird hacks for embedded work!

cc @japaric

[hanna-kruppe]

Just for the record: in #45029 @sunfishcode brought up the possibility of other languages unwinding through Rust code. IIUC this could theoretically work if the Rust code contains no landing pads, though I believe that it is and should be UB instead. Still, I find it interesting that if we wanted to support that, the change proposed here would break that (b/c any unwinding would now be UB, rather than passing through the landing-pad-free code harmlessly).

[alexcrichton]

@rkruppe that seems like support for that would be opt-in, if ever?

[hanna-kruppe]

Mayyyyybe? I don't think this is likely to happen regardless of the details. I mostly wanted to record the thought somewhere.

[retep998]

There are currently platforms where Rust and C++ can unwind through each other and landing pads will function correctly (such as windows msvc). I don't understand why anyone would want to support unwinding between Rust and other languages when -C panic=abort is specified, considering the whole point of -C panic=abort is that no unwinding occurs.