[WebAssembly] Make pthread imply bulk-memory, mutable-globals
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
Differential Revision: https://reviews.llvm.org/D64586