Expose the offset of the modrm byte

This is useful for applications rewriting instructions, for example [Frida](https://github.com/frida/frida-python/wiki/Tutorial:-Inspecting-and-Manipulating-Function-Calls-\(on-Unix\))'s [X86Relocator](https://github.com/frida/frida-gum/blob/master/gum/arch-x86/gumx86relocator.c#L515).