Auto merge of #103894 - mati865:gnullvm-libunwind-changes, r=thomcc

Change the way libunwind is linked for *-windows-gnullvm targets

I have no idea why previous way works for `x86_64-fortanix-unknown-sgx` (assuming it actually works...) but not for `gnullvm`. It fails when linking libtest during Rust build (unless somebody adds `RUSTFLAGS='-Clinkarg=-lunwind'`).
Also fixes exception handling on AArch64.

Author: bors