[WebAssembly] Implement __builtin_return_address for emscripten

Summary:
In this patch, `ISD::RETURNADDR` is lowered on the emscripten target
to the new Emscripten runtime function `emscripten_return_address`, which
implements the functionality.

Patch by Guanzhong Chen

Reviewers: tlively, aheejin

Reviewed By: tlively

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62210

llvm-svn: 361454