[wasm-mt] place multi-threaded wasm build into separate artifacts folders

[lambdageek]

Build artifacts for the /p:WasmEnableThreads=true should go into a separate output folder. In other words it should be possible to build with /p:WasmEnableThreads=false in the same tree and without the build artifacts interfering with each other.

Needed by #68508

Part of #68162

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Issue Details

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Author:	lambdageek
Assignees:	-
Labels:	untriaged, area-Infrastructure-mono
Milestone:	-

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Issue Details

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Build artifacts for the /p:WasmEnableThreads=true should go into a separate output folder. In other words it should be possible to build with /p:WasmEnableThreads=false in the same tree and without the build artifacts interfering with each other.

Part of #68162

Author:	lambdageek
Assignees:	-
Labels:	arch-wasm, untriaged, area-Infrastructure-mono
Milestone:	-

[lambdageek]

@steveisok @akoeplinger The initial wasm-mt PR landed; this is now the next issue for the runtime pack

[steveisok]

@lambdageek Alex is working on moving cmake related items out of mono.proj and into its own file. That'll make it easier to build the runtime twice w/o a bunch of conditions / hacks.

[lambdageek]

Note that this needs to be somewhat general: for the /p:WasmEnablePerfTracing=true build we should also place its artifacts into an ep subfolder (tracking: #69268)

[lambdageek]

This is done using MonoWasmBuildVariant=perftrace or MonoWasmBuildVariant=multithread in #71054 which also published runtime packs. (we decided to go with separate runtime packs for each variant)