Stop inferring the `PlatformTarget` in the SDK when targeting modern .NET applications

[baronfel]

          Also, you may want to stop inferring the `PlatformTarget` in the SDK when targeting modern .NET applications: 

sdk/src/Tasks/Microsoft.NET.Build.Tasks/targets/Microsoft.NET.RuntimeIdentifierInference.targets

Lines 124 to 157 in 88c6473

	<!-- Determine PlatformTarget (if not already set) from runtime identifier. -->
	<Choose>
	<When Condition="'$(PlatformTarget)' != '' or '$(RuntimeIdentifier)' == ''" />
	<When Condition="$(RuntimeIdentifier.EndsWith('-x86')) or $(RuntimeIdentifier.Contains('-x86-'))">
	<PropertyGroup>
	<PlatformTarget>x86</PlatformTarget>
	</PropertyGroup>
	</When>
	<When Condition="$(RuntimeIdentifier.EndsWith('-x64')) or $(RuntimeIdentifier.Contains('-x64-'))">
	<PropertyGroup>
	<PlatformTarget>x64</PlatformTarget>
	</PropertyGroup>
	</When>
	<When Condition="$(RuntimeIdentifier.EndsWith('-arm')) or $(RuntimeIdentifier.Contains('-arm-'))">
	<PropertyGroup>
	<PlatformTarget>arm</PlatformTarget>
	</PropertyGroup>
	</When>
	<When Condition="$(RuntimeIdentifier.EndsWith('-arm64')) or $(RuntimeIdentifier.Contains('-arm64-'))">
	<PropertyGroup>
	<PlatformTarget>arm64</PlatformTarget>
	</PropertyGroup>
	</When>
	<Otherwise>
	<PropertyGroup>
	<PlatformTarget>AnyCPU</PlatformTarget>
	</PropertyGroup>
	</Otherwise>
	</Choose>

Originally posted by @jkotas in #42556 (comment)

[marcpopMSFT]

Triage: The safest solution here is to just add a When after the first one that checks if it's .net core and defaults to AnyCPU. We would need to confirm that cppcli is excluded from that condition. That solution shouldn't require other changes elsewhere.

[baronfel]

@joeloff noted that the platform bit in the PE Header may influence registry hive/program files and other defaults - this could be impactful for already-compiled applications.

[marcpopMSFT]

Also watch out for what value a solution sets (is it platform rather than platformtarget) and if it's set by the time this code is evaluated.

[baronfel]

Also watch out for what value a solution sets (is it platform rather than platformtarget) and if it's set by the time this code is evaluated.

The Platform<->PlatformTarget relationship is handled by the SDK. The Solution sets Platform for the project in the generated metaproj, and the SDK translates that to a PlatformTarget value here. This value then feeds into the logic in RuntimeIdentifierInference afterwards.

[jkotas]

@joeloff noted that the platform bit in the PE Header may influence registry hive/program files and other defaults

That's true for .NET Framework. I do not think it is the case for .NET Core.