Unspecified Platform value 'windows-arm-v7' added to the targetPlatform valueset.

Hi

With a build.gradle script defining the platform dimension as
```
platforms {
win32 {operatingSystem "windows" architecture 'x86' }
win64 {operatingSystem "windows" architecture 'x64' }
linux32 {operatingSystem "linux" architecture 'x86' }
linux64 {operatingSystem "linux" architecture 'x64' }
macos {operatingSystem "macosx" architecture 'x64' }
}
```
the targetPlatform valueset contains the unwanted/unspecified `windows-arm-v7` which implies a exception when the platform name is controled by the script.

Execution of this build.gradle script
[build.gradle.txt](https://github.com/gradle/gradle-native/files/1662939/build.gradle.txt)

```
C:\Users\fandre\Documents\git\gradle\subprojects\docs\src\samples\native-binaries\prebuilt>C:\ProgramFiles\Gradle\gradle-4.5-rc-1\bin\gradle clean build
Platform=platform 'linux32'
Platform=platform 'linux32'
Platform=platform 'linux64'
Platform=platform 'linux64'
Platform=platform 'macos'
Platform=platform 'macos'
Platform=platform 'win32'
Platform=platform 'win32'
Platform=platform 'win64'
Platform=platform 'win64'
Platform=platform 'windows_x86'
Platform=platform 'windows_x86'
Platform=platform 'windows_x86-64'
Platform=platform 'windows_x86-64'
Platform=platform 'windows_arm-v7'

FAILURE: Build failed with an exception.

* Where:
Build file 'C:\Users\fandre\Documents\git\gradle\subprojects\docs\src\samples\native-binaries\prebuilt\build.gradle' line: 11

* What went wrong:
Could not determine the dependencies of task ':linkMainReleaseExecutable'.
> Unknown architecture: arm-v7
```

Start by reproducing the failure with the sample at subprojects/docs/src/samples/native-binaries/prebuilt/build.gradle, especially line 11, and inspect how the targetPlatform valueset is assembled. Done means the explicitly defined platforms no longer include windows_arm-v7 and the sample build completes without the unknown architecture error.

Written by the indexing model from the issue text.