VsTest does not correctly pick the testhost for ARM64 native assemblies

[drognanar]

The method GetArchitectureForSource is meant to get the dll architecture for native assemblies.
However, that method does not support the ARM64 architecture. Looking at MSBuild defines the following constant internal const UInt16 IMAGE_FILE_MACHINE_ARM64 = 0xAA64; // ARM64 Little-Endian.

We should also add the support for this image number.

The current behaviour is that vstest does not know the architecture of the child process and in effect it picks either the user specified default or the machine's architecture.