[AVR] standard library support for AVR

- when compiling embedded cross compile target standard libraries, include AVR
- add 16-bit pointer as a conditional compilation condition and get the void pointer size right for gyb sources
- attempt to fix clang importer not importing __swift_intptr_t correctly on 16 bit platforms
- changed the unit test target to avr-none-none-elf to match the cmake build

[AVR] got the standard library compiling in a somewhat restricted form:

General
- updated the Embedded Runtime
- tweaked CTypes.swift to fix clang import on 16 bit platforms

Strings
- as discussed in https://forums.swift.org/t/stringguts-stringobject-internals-how-to-layout-on-16-bit-platforms/73130, I went for just using the same basic layout in 16 bit as 32 bit but with 16 bit pointers/ints... the conversation is ongoing, I think something more efficient is possible but at least this compiles and will probably work (inefficiently)

Unicode
- the huge arrays of unicode stuff in UnicodeStubs would not compile, so I skipped it for AVR for now.

Synchronization
- disabled building the Synchronization library on AVR for now. It's arguable if it adds value on this platform anyway.

stdlib/public/core/EmbeddedRuntime.swift

@@ -41,9 +41,12 @@ public struct HeapObject {
 #if _pointerBitWidth(_64)
   static let doNotFreeBit = Int(bitPattern: 0x8000_0000_0000_0000)
   static let refcountMask = Int(bitPattern: 0x7fff_ffff_ffff_ffff)
-#else
+#elseif _pointerBitWidth(_32)
   static let doNotFreeBit = Int(bitPattern: 0x8000_0000)
   static let refcountMask = Int(bitPattern: 0x7fff_ffff)
+#elseif _pointerBitWidth(_16)
+  static let doNotFreeBit = Int(bitPattern: 0x8000)
+  static let refcountMask = Int(bitPattern: 0x7fff)
 #endif
 
   // Note: The immortalRefCount value of -1 is also hard-coded in IRGen in `irgen::emitConstantObject`.
@@ -55,8 +58,10 @@ public struct HeapObject {
 
 #if _pointerBitWidth(_64)
   static let bridgeObjectToPlainObjectMask = UInt(0x8fff_ffff_ffff_fff8)
-#else
+#elseif _pointerBitWidth(_32)
   static let bridgeObjectToPlainObjectMask = UInt(0xffff_ffff)
+#elseif _pointerBitWidth(_16)
+  static let bridgeObjectToPlainObjectMask = UInt(0xffff)
 #endif
 }