[WebAssembly] Make pthread imply bulk-memory, mutable-globals

Summary:
This paves the way for using passive segments in pthread builds, which
will make separate memory files unnecessary. Mutable globals are also
necessary for the upcoming implementation of TLS.

Reviewers: aheejin, dschuff

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64586

llvm-svn: 365935