Fuchsia: Reland enable observatory. Pass rpath for host builds.

runtime/observatory/BUILD.gn

Index: runtime/observatory/BUILD.gn
diff --git a/runtime/observatory/BUILD.gn b/runtime/observatory/BUILD.gn
index c55b92d9143acca3fa0a99e83a841e314e1e29f7..ca85f297088b11ff6be4cc65960dae733f024c87 100644
--- a/runtime/observatory/BUILD.gn
+++ b/runtime/observatory/BUILD.gn
@@ -23,7 +23,8 @@ declare_args() {
 }
 
 # We set this to "" in the Fuchsia build to force building with dart_bootstrap.
-if (defined(is_fuchsia) && is_fuchsia) {
+if (defined(is_fuchsia) && defined(is_fuchsia_host) &&
+    (is_fuchsia || is_fuchsia_host)) {
   dart_host_pub_exe = ""
 }