Get the renamed Platform passed in from command-line

### Steps to reproduce
It is allowed to renamed a platform from "AnyCPU, x86, x64" to a customized name for a solution, such that we can build with a specific platform with command like:

`msbuild MSBuildTests.sln -p:Platform=MyPlatform1;Configuration=MyConfig1`

We can get MyConfig1 from `$(Configuration)` in a `.csproj`, However, there is no way to get the `MyPlatform1` argument from `$(Platform)` as it is overwritten by `x64` (assume it is renamed from x64).

There should be a way to get the renamed platform that passed in from command-line when we build on a `.sln`. Or we already have it?

Project file
```xml

Exe
netcoreapp2.1
AnyCPU;x64

```

Directory contents:
```
/
- MSBuildTests.sln
- MSbuildTests/
- c.cs
- MSBuildTests.csproj
```

Command line
```
`msbuild MSbuildTests.sln -p:platform=MyPlatform1 -t:GetRenamedPlatform`
```
### Expected behavior
Print "MyPlatform1"

### Actual behavior
Print "x64"

### Environment data
`msbuild /version` output:
Microsoft (R) Build Engine version 16.2.37902+b5aaefc9f for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

16.2.37902.0