[NativeAOT-LLVM] Upgrade emscripten and workaround cache lock problem

[yowl]

This PR upgrades the Emscripten version and puts a workaround for the cache lock problem we've been experiencing. I've not got to the bottom of the underlying reason, but the workaround seems ok locally (and it was failing locally before).

Workaround is to build the Emscripten ports for ICU and zlib upfront. These builds are cached, so it does not incur extra build work.

emscripten-core/emscripten#15440

Also suppress warning for __x variables as latest clang defines them as reserved identifiers.

[SingleAccretion]

I think we need to add TODOs to remove the supressions once dotnet/runtime@2265a61 is merged (which also suppresses the warnings, but is present in the upstream).

[yowl]

Linux thread test failed with

native/BasicThreading ''
FinalizeTest failed
Expected: 100
Actual: 255

Seems unrelated.

[jkotas]

Thanks