Handle removal of `core::intrinsics::abort()` on nightly.

Now uses autocfg so build won't fail when nightly functions are removed in the future.
We migrated to the replacement function core::process::abort_immediate() (still nightly).

Changes

• Prefer core::process::abort_immediate() to core::arch::wasm64::unreachable() (tnyxtmnq)

Fixes

• Use autocfg to check if nightly features work before using them (wulnptwt)
  • Avoids build failures caused by the removal of nightly functions.
• Document use of unwinding past extern "C" on 1.81 (qmvsysnu)
• On nightly, use core::process::abort_immediately() instead of core::intrinsics::abort() (mxmspryo)
  • The latter has been removed on recent nightly versions.