[ARM] Why MI thunks are always implemented in ARM code?

[Doug Kwan (關振德)]

I saw that MI thunks are always issued in ARM mode. Is there a
specific reason? I browse the EABI and its supplement and there is no
mentioning of how thunks are implemented.  I think I can replace a
20-byte ARM MI thunk with a 4 bytes THUMB equivalent when the thunkee
is also in THUMB mode.  Is there any problem with this?

-Doug