Support zero and sign extension for ARMv7

[sarranz]

This doesn't work

export fn main() -> reg u32 {
    reg u16 x;
    x = 0;
    reg u32 r;
    r = (32u)x;
    return r;
}

We need to use the UBFX and SBFX instructions that are already implemented in the compiler,. The problem is the U32 restriction for all arm related things.

[sarranz]

We can do this with the instructions from #805, I don't know if there's any reason to prefer one or the other.
Edit: it looks like UXTH/UXTB have a smaller encoding than UBFX, but in general we always want to be 32bit aligned and in this case it doesn't make any difference.