Can't distinguish ARM and x64 layers on the same system

**Describe the bug**
With #1844, the loader now gives this warning on ARM windows:

**Environment (please complete the following information):**
- OS: Windows - ARM
- Bitdepth: [64-bit]
- GPU: N/A
- Graphics Driver: N/A
- SDK or header
- Enabled layers: VK_LAYER_MSFT_driver_sorting

Since MSFT provides both a x64 and an ARM layer, the loader uses loading of the dll to determine which to load. With the de-duplication happening based on the name alone, this leads to the possibility of the wrong layer being chosen.

The layer manifest "arch" field should have "arm" added as a way to distinguish between the layers.

Start by reading the loader's layer-manifest handling and the de-duplication behavior associated with #1844. Check how the MSFT x64 and ARM manifests are selected on Windows ARM, then verify that the manifest arch field distinguishes both layers without choosing the wrong one.

Written by the indexing model from the issue text.