Crossgen2 does not emit runtime-async methods for ARM32 targets

[jtschuster]

Crossgen2 does not properly emit runtime-async methods for ARM32. I think it's because relocs in ARM32 require the thumb bit to be set, but it isn't properly done for the async methods. I attempted to solve the issue in c0f9dbd,
but decided to just punt and solve it in a separate PR.