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

[mati865]

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.

[rustbot]

r? @thomcc

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[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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[thomcc]

This looks fine to me.

@bors r+

[bors]

📌 Commit 7333ee0 has been approved by thomcc

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7333ee0 with merge 7b513af...

[bors]

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 7b513af to master...

[rust-timer]

Finished benchmarking commit (7b513af): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.6%	[-5.6%, -3.6%]	2
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.2%	[-4.2%, -4.2%]	1
All ❌✅ (primary)	-	-	0