[WebAssembly] -pthread implies -target-feature +sign-ext

Summary:
The sign extension proposal was motivated by a desire to not have
separate sign-extending atomic operations, so it is meant to be
enabled when threads are used.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, sunfish, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69075

llvm-svn: 375199