[WebAssembly] Temporarily disable bulk-memory with -pthread

Summary:
To prevent the instability of bulk-memory in the wasm backend from
blocking separate pthread testing, temporarily remove the logic that
adds -mbulk-memory in the presence of -pthread. Since browsers will
ship bulk memory before or alongside threads, this change will be
reverted as soon as bulk memory has stabilized in the backend.

Reviewers: sbc100

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58854

llvm-svn: 355248