Use panic=abort instead of panic=unwind

[jrmuizel]

This is the same mode as Gecko uses and will get better codegen
once rust-lang/rust#45012 has landed.

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This change is 

[nox]

What does that imply for other users of webrender?

[jrmuizel]

They will get worse performance until Rust supports emitting noalias metadata in code that can unwind. Although even then, having to unwind will always constrain the optimizer.

[Gankra]

I think the top crate wins, so servo will choose to ignore this directive. However this will make wrench use panic=abort.

[Gankra]

Actually I think servo/gecko won't see this at all, since it's in the workspace, and not the crates.

[nox]

@gankro Ok, I thought that this would set panic=abort for any user of this crate, and thus that any panic in WR would have made their program abort. That would have been a blocker in my book, but it seems I was wrong anyway so disregard me.

[Gankra]

Yeah I totally get it, but it should be fine?

[glennw]

I confirmed in a local servo build that a panic in WR doesn't abort the process with this change, so I'm assuming that means it's OK to merge.

@bors-servo r+

[bors-servo]

📌 Commit 6af8c83 has been approved by glennw

[bors-servo]

⌛ Testing commit 6af8c83 with merge 16251d8...

[bors-servo]

☀️ Test successful - status-appveyor, status-travis
Approved by: glennw
Pushing 16251d8 to master...