[Support] Fix build under Emscripten

[loladiro]

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).

lib/Support/Unix/Path.inc

default:
     return true;
   }
 #elif defined(__CYGWIN__)
   // Cygwin doesn't expose this information; would need to use Win32 API.
   return false;
 #elif defined(__Fuchsia__)
   // Fuchsia doesn't yet support remote filesystem mounts.
   return true;
+#elif defined(__EMSCRIPTEN__)
+  // Emscripten doesn't currently support remote filesystem mounts.
+  return true;
 #elif defined(__HAIKU__)
   // Haiku doesn't expose this information.
   return false;
 #elif defined(__sun)
   // statvfs::f_basetype contains a null-terminated FSType name of the mounted target
   StringRef fstype(Vfs.f_basetype);
   // NFS is the only non-local fstype??
   return !fstype.equals("nfs");