[release/6.0][wasm] Change dotnet.wasm link optimization

[radekdoulik]

Context: #60349

When we switched to Emscripten 2.0.21, we stopped using deprecated
--llvm-opts 2 option. This caused unwanted performance degradation.
Switching to -O2 get us similar performance as with --llvm-opts.

Blazor wasm test app Time to first UI benchmark times:

branch/commit link option time dotnet.wasm size .br size
--------------------+------------+--------+------------------+---------
release/6.0 92ff02 -Oz 491ms 2,430,639 850,109
release/6.0 92ff02 -O2 457ms 2,474,518 853,765
release/6.0 92ff02 -O3 444ms 2,555,824 857,961

Customer Impact

Fixes a startup time regression for blazorwasm between .NET 5 an .NET 6 by a reverting compiler option change that was accidentally introduced as part of other build changes.

Testing

Manual and Automated

Regression

Yes, startup time regression.

Risk

Low, switch to compiler options we're using in main.

[lewing]

we're going to let this bake for a bit before we take it in servicing

[ghost]

Tagging subscribers to 'arch-wasm': @lewing
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Context: #60349

When we switched to Emscripten 2.0.21, we stopped using deprecated
--llvm-opts 2 option. This caused unwanted performance degradation.
Switching to -O2 get us similar performance as with --llvm-opts.

Blazor wasm test app Time to first UI benchmark times:

branch/commit link option time dotnet.wasm size
--------------------+------------+--------+-----------------
release/6.0 92ff02 -Oz 491ms 2,430,639
release/6.0 92ff02 -O2 457ms 2,474,518
release/6.0 92ff02 -O3 444ms 2,555,824

Author:	radekdoulik
Assignees:	-
Labels:	arch-wasm, area-Build-mono
Milestone:	-

[Anipik]

@marek-safar@radical can you review this one ?

[radical]

LGTM. but I would also wait for an approval from @lewing