py/inlineasm: Add ability to specify return type of asm_thumb funcs.

Supported return types are: object, bool, int, uint.

For example:

@micropython.asm_thumb
def foo(r0, r1) -> uint:
    add(r0, r0, r1)

py/qstrdefs.h

@@ -113,6 +113,7 @@ Q(asm_thumb)
 Q(label)
 Q(align)
 Q(data)
+Q(uint)
 #endif
 
 Q(builtins)