[WebAssembly] Make driver -pthread imply linker --shared-memory

Summary:
This eliminates a linker error the user might otherwise see about how
using the 'atomics' feature requires --shared-memory.

Reviewers: sbc100, aheejin

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59721

llvm-svn: 356817