Simplify inline exit frames.

[dvander]

When invoking C++ from JIT code, we need to construct an "exit frame"
so that FrameIterator can recover a stack trace. The normal way of
creating an exit frame is implicitly, via a call. The call pushes a
return address, and the new prologue configures the frame pointer and
stores it in the Environment.

Notably, this means there is a small inline helper function every time
we transition to C++. For performance-sensitive code, like native calls,
this is not ideal. Instead we create an "inline" exit frame. Instead of
calling, we derive the return address at compile-time, push it, and thus
avoid the call instructions.

This patch makes additional improves to the inline-exit path by also
eliding "leave" instructions. It also preserves ebp, which should
improve stack walking by Breakpad.

[asherkin]

This has indeed massively improved breakpad stack walking, and the stack layout is a lot easier to grok, thanks! Tested the new unbound native error path and that all seems fine too.

Does the x64 JIT need the same additions before this can be merged? I'll take a look into the Emscripten build failure.

[dvander]

re: x64 - no JIT yet, so I just removed some unused functions.