[wasm-mt] fix build config logic error, and pthread_getschedparam initialization

[lambdageek]

Two followups for the initial merge PR.

1. On Emscripten, pthread_getschedparam doesn't write to its output variables, so set them to good initial values
2. The wasm CMakeLists.txt defaults DISABLE_THREADS to ON, so instead set it to off when MonoWasmThreads is true, and leave it at the default value (ie off) otherwise.

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Issue Details

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Two followups for the initial merge PR.

1. On Emscripten, pthread_getschedparam doesn't write to its output variables, so set them to good initial values
2. The wasm CMakeLists.txt defaults DISABLE_THREADS to ON, so instead set it to off when MonoWasmThreads is true, and leave it at the default value (ie off) otherwise.

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

[akoeplinger]

On Emscripten, pthread_getschedparam doesn't write to its output variables, so set them to good initial values

Should we file an upstream issue for that? sounds unexpected

[lambdageek]

Should we file an upstream issue for that? sounds unexpected

Maybe. I'm not sure they can make the function return some sensible values without having to implement a bunch of bookkeeping to try and match the POSIX spec.