- this cleans up `Utils.RunProcess` a bit, but doesn't change the
  behavior for the existing non-wasm users (Apple, and Android build
  tasks)

- New overloads are added to support getting the output, and exit code,
  and throwing only when really needed
- And running shell commands, appropriately on windows, and unix.

Currently, we use `mono-aot-cross` to compile the managed assemblies
into bitcode files. Then we pass the `.bc` files to `emcc` for linking.
We can save some amount of work, if precompile `.bc` files to `.o`
files, and pass them for linking.

This would also allow us to rebuild the `.o` files only when needed,
thus improving incremental builds.
Also, emscripten has recently removed some amount of parallelization, so
this allows us to do some pre-linking work in parallel, and speed up the
builds a little bit.

This is implemented via a new `EmccCompile` task, that is meant *only*
for compiling, and not as a general purpose `emcc` invocation task.

`System.IO.MemoryMappedFiles.Tests` - Issue: dotnet#54194

- `IBuildEngine9.RequestCores`
- This is available only in 16.10+, so bump msbuild used for building
  the tests

…third

- This is meant to be disabled for CI (it has `IgnoreForCI=true` in the
  csproj)

.. because we keep running into the following on browser, and other
platforms:

```
/__w/1/s/src/libraries/Microsoft.NETCore.Platforms/tests/Microsoft.NETCore.Platforms.Tests.csproj : error NU1201: Project System.Security.AccessControl is not compatible with net472 (.NETFramework,Version=v4.7.2). Project System.Security.AccessControl supports: net6.0 (.NETCoreApp,Version=v6.0) [/__w/1/s/Build.proj]
\#\#[error]src/libraries/Microsoft.NETCore.Platforms/tests/Microsoft.NETCore.Platforms.Tests.csproj(0,0): error NU1201: (NETCORE_ENGINEERING_TELEMETRY=Restore) Project System.Security.AccessControl is not compatible with net472 (.NETFramework,Version=v4.7.2). Project System.Security.AccessControl supports: net6.0 (.NETCoreApp,Version=v6.0)
```

This is already not being sent to CI with `IgnoreForCI=true`.
It fails on CI with:

```
/_/src/Shared/NativeMethodsShared.cs(855,29): error IL1005: Microsoft.Build.Shared.NativeMethodsShared.SystemInformation.get: Error processing method 'Microsoft.Build.Shared.NativeMethodsShared.SystemInformationData.SystemInformationData()' in assembly 'Microsoft.Build.Utilities.Core.dll' [/__w/1/s/src/libraries/Microsoft.NETCore.Platforms/tests/Microsoft.NETCore.Platforms.Tests.csproj]
\##[error]/_/src/Shared/NativeMethodsShared.cs(855,29): error IL1005: (NETCORE_ENGINEERING_TELEMETRY=Build) Microsoft.Build.Shared.NativeMethodsShared.SystemInformation.get: Error processing method 'Microsoft.Build.Shared.NativeMethodsShared.SystemInformationData.SystemInformationData()' in assembly 'Microsoft.Build.Utilities.Core.dll'
  Mono.Linker.LinkerFatalErrorException: /_/src/Shared/NativeMethodsShared.cs(855,29): error IL1005: Microsoft.Build.Shared.NativeMethodsShared.SystemInformation.get: Error processing method 'Microsoft.Build.Shared.NativeMethodsShared.SystemInformationData.SystemInformationData()' in assembly 'Microsoft.Build.Utilities.Core.dll'
   ---> System.NotImplementedException: switch
     at Mono.Linker.Steps.UnreachableBlocksOptimizer.BodyReducer.IsConstantBranch(OpCode opCode, Int32 operand) in illink.dll:token 0x6000770+0x41
     at Mono.Linker.Steps.UnreachableBlocksOptimizer.BodyReducer.RemoveConditions() in illink.dll:token 0x6000768+0x22a
     at Mono.Linker.Steps.UnreachableBlocksOptimizer.BodyReducer.RewriteBody() in illink.dll:token 0x6000766+0xa
     at Mono.Linker.Steps.UnreachableBlocksOptimizer.ProcessStack() in illink.dll:token 0x60004e9+0x102
     at Mono.Linker.Steps.UnreachableBlocksOptimizer.ProcessMethod(MethodDefinition method) in illink.dll:token 0x60004e5+0x44
     at Mono.Linker.Steps.MarkStep.ProcessMethod(MethodDefinition method, DependencyInfo& reason, Scope& scope) in illink.dll:token 0x600040e+0x9b
     at Mono.Linker.Steps.MarkStep.ProcessQueue() in illink.dll:token 0x6000399+0x24
     --- End of inner exception stack trace ---
     at Mono.Linker.Steps.MarkStep.ProcessQueue() in illink.dll:token 0x6000399+0xaa
     at Mono.Linker.Steps.MarkStep.ProcessPrimaryQueue() in illink.dll:token 0x6000396+0xa
     at Mono.Linker.Steps.MarkStep.Process() in illink.dll:token 0x6000393+0x0
     at Mono.Linker.Steps.MarkStep.Process(LinkContext context) in illink.dll:token 0x600038a+0x41
     at Mono.Linker.Pipeline.ProcessStep(LinkContext context, IStep step) in illink.dll:token 0x6000264+0x0
     at Mono.Linker.Pipeline.Process(LinkContext context) in illink.dll:token 0x6000263+0xf
     at Mono.Linker.Driver.Run(ILogger customLogger) in illink.dll:token 0x60000fd+0x20
  Optimizing assemblies for size, which may change the behavior of the app. Be sure to test after publishing. See: https://aka.ms/dotnet-illink
/__w/1/s/.dotnet/sdk/6.0.100-preview.4.21255.9/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.ILLink.targets(80,5): error NETSDK1144: Optimizing assemblies for size failed. Optimization can be disabled by setting the PublishTrimmed property to false. [/__w/1/s/src/libraries/Microsoft.NETCore.Platforms/tests/Microsoft.NETCore.Platforms.Tests.csproj]
\##[error].dotnet/sdk/6.0.100-preview.4.21255.9/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.ILLink.targets(80,5): error NETSDK1144: (NETCORE_ENGINEERING_TELEMETRY=Build) Optimizing assemblies for size failed. Optimization can be disabled by setting the PublishTrimmed property to false.
```

Issue: dotnet#54194