[WebAssembly] Add multivalue and tail-call target features

Summary:
These features will both be implemented soon, so I thought I would
save time by adding the boilerplate for both of them at the same time.

Reviewers: aheejin

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D62047

llvm-svn: 361516