Implement function prologues in ARM

Implement function prologues in ARM

This is mainly what's holding us back in test/quick. There's a lot of
stuff going on here:

- Adding code-gen-helpers-arm.cpp. There's a practical need for this:
  we're starting to write out a lot of calls to C++ functions, which
  should be abstracted out because it happens differently depending on
  whether we're simulating ARM or running on native ARM.

- Initialize rStashedAR when starting the simulator. This was a bug.

- Change the order of pushing x29 and x30 around calls to properly mimic
  the x64 stack frame.

- Implement fcallHelperThunk.

- Templatize a couple of vixl functions to let us move pointers into
  registers without reinterpret_cast. Also fill the simulator stack with
  junk before starting up a simulator; in an early version of this diff
  we were actually reading from the stack out-of-bounds. It wasn't
  causing any bugs, but let's get out ahead of that.

Reviewed By: @jdelong

Differential Revision: D1019980