[Support] Fix build under Emscripten

Summary:
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).

Reviewers: kripken
Differential Revision: https://reviews.llvm.org/D63688

llvm-svn: 364143