Use new panic-strategy field in custom target?

[phil-opp]

PR rust-lang/rust#36794 introduced a new panic-strategy field for target JSON files, that allows to set the default panic strategy for the target. We could use it to get rid of the profile options in the Cargo.toml. This would also remove the need for eh_personality and _Unwind_Resume (since the sysroot is then compiled without unwinding support too).

However, we couldn't introduce it before the Returning from Exceptions post, since we're using the default x86_64-unknown-linux-gnu target in the earlier posts.

[phil-opp]

Fixed in #310