Only add CFI instructions for panic=unwind build

[nbdd0121]

Fix #39

CFI directives are omitted when panic=abort is used.

[nbdd0121]

@bjorn3 @sunfishcode would you mind doing a quick review? Thanks!

[sunfishcode]

This looks good, thanks! I was worried fixing this would require duplicating a bunch of code, but this macro approach looks reasonable. I can also confirm that this works in the Eyra build.

[bjorn3]

I was thinking if LTO could cause problems when unwinding is compiled as panic=unwind and the executable as panic=abort, but then I remembered that we no longer make all functions panic=abort when doing LTO when any crate is panic=abort to handle extern "C-unwind" safely.