[WebAssembly] Check bulk-memory when adjusting lang opts

We previously had logic to disable pthreads, set the ThreadModel to Single, and
disable thread-safe statics when the atomics target features is disabled, since
that means that the resulting program will not be used in a threaded context.
Similarly check for the presence of the bulk-memory feature, since that is also
necessary to produce multithreaded programs.

Differential Revision: https://reviews.llvm.org/D121014