[WebAssemby] Enable "-mthread-model single" by default, for now.

The WebAssembly standard does not yet have threads, and while it's in the
process of being standardized, it'll take some time for it to make it
through and be available in all popular implementations. With increasing
numbers of people using the LLVM wasm backend through LLVM directly rather
than through Emscripten, it's increasingly important to have friendly
defaults.

See also https://bugs.llvm.org/show_bug.cgi?id=35411

llvm-svn: 319101