"Platform target" does not include "ARM64" as an option. (2)

[tmeschter]

Visual Studio Version:

16.10 Preview 2 (around 31202.13.main)

Summary:

The "Platform Target" property does not surface ARM or ARM64:

Steps to Reproduce:

1. Open Tools | Options | Environment | Preview Features and ensure that "Enable Updated Project Properties UI" is checked.
2. Create a new C# console application targeting .NET, .NET Core, or .NET Standard (not .NET Framework).
3. Right-click on the project in Solution Explorer and choose "Properties".
4. Go to the Build page and examine the "Platform target" drop down.

Expected Behavior:

ARM64 (and possibly ARM) are displayed.

Actual Behavior:

Only "Any CPU", x64, and x86 are shown.

User Impact:

User cannot use the property pages to target the ARM64 architecture.

Additional Notes

Currently the list of platforms is hard-coded here:

project-system/src/Microsoft.VisualStudio.ProjectSystem.Managed/ProjectSystem/Rules/PropertyPages/BuildPropertyPage.xaml

Lines 39 to 50 in 20d7f46

	<EnumProperty Name="PlatformTarget"
	DisplayName="Platform target"
	Description="Specifies the processor to be targeted by the output file. Choose 'x86' for any 32-bit Intel-compatible processor, choose 'x64' for any 64-bit Intel-compatible processor, or choose 'Any CPU' to specify that any processor is acceptable. 'Any CPU' is the default value for projects, because it allows the application to run on the broadest range of hardware."
	HelpUrl="https://go.microsoft.com/fwlink/?linkid=2147129"
	Category="General">
	<EnumValue Name="AnyCPU"
	DisplayName="Any CPU" />
	<EnumValue Name="x64"
	DisplayName="x64" />
	<EnumValue Name="x86"
	DisplayName="x86" />
	</EnumProperty>

.

We should instead make this a dynamic enum, and create a dynamic enum value provider that surfaces the set of platforms in the <AvailablePlatforms> property.