[WebAssembly] WebAssemblyLowerEmscriptenEHSjLj: use getter/setter for accessing tempRet0

Rather than assuming that `tempRet0` exists in linear memory only assume
the getter/setter functions exist.  This avoids conflicting with
binaryen which declares a wasm global for this purpose and defines it's
own getter and setter for that.

The other advantage of doing things this way is that it leaving
it up to the linker/finalizer to decide how to actually store this
temporary.  As it happens binaryen uses a wasm global which is more
appropriate since it is thread safe.

This also allows us to change the way this is stored in the future
(memory, TLS memory, wasm global) without modifying LLVM.

This is part of a 4 part change:
LLVM: https://reviews.llvm.org/D53240
fastcomp: https://github.com/kripken/emscripten-fastcomp/pull/237
emscripten: https://github.com/kripken/emscripten/pull/7358
binaryen: https://github.com/WebAssembly/binaryen/pull/1709

Differential Revision: https://reviews.llvm.org/D53240

llvm-svn: 347340