[Support] Fix build under Emscripten
Emscripten's libc doesn't define MNT_LOCAL, thus causing a build
failure in the fallback path. However, to the best of my knowledge,
it also doesn't support remote file system mounts, so we may simply
return true here (as we do for e.g. Fuchsia). With this fix, the
core LLVM libraries build correctly under emscripten (though some
of the tools and utils do not).
Differential Revision: https://reviews.llvm.org/D63688