WebAssembly: emit `(func (param t) (result t))` s-expressions

Description

WebAssembly: emit (func (param t) (result t)) s-expressions

Summary: Match spec format: https://github.com/WebAssembly/spec/blob/master/ml-proto/test/fac.wasm

Reviewers: sunfish

Subscribers: llvm-commits, jfb

Differential Revision: http://reviews.llvm.org/D12307