Add Embedded Swift support

Allow compilation of the `Bluetooth` library target for the [Embedded Swift](https://github.com/swiftlang/swift-evolution/blob/main/visions/embedded-swift.md) language mode.

- [x] Compile `Bluetooth` module for Embedded Swift
- [x] Compile `BluetoothGAP` module for Embedded Swift
- [ ]  Compile `BluetoothGATT` module for Embedded Swift
- [ ] Compile `BluetoothHCI` module for Embedded Swift