Reland "Make compiler/optimizing/ symbols hidden."

This reverts commit 0a51605ddd81635135463dab08b6f7c21b58ffb0.

Reason for revert: Reland after some of the required work
was merged in other CLs.

Also address a TODO from the original CL to mark required
symbols with EXPORT in `intrinsic_objects.h`.

Also mark symbols in new files as HIDDEN.

Bug: 186902856
Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I936d448983928af23614ca82c2d0bf9a645e2c52
diff --git a/compiler/optimizing/intrinsics.cc b/compiler/optimizing/intrinsics.cc
index 0feb92d..aa5e4f3 100644
--- a/compiler/optimizing/intrinsics.cc
+++ b/compiler/optimizing/intrinsics.cc
@@ -32,7 +32,7 @@
 #include "scoped_thread_state_change-inl.h"
 #include "thread-current-inl.h"
 
-namespace art {
+namespace art HIDDEN {
 
 std::ostream& operator<<(std::ostream& os, const Intrinsics& intrinsic) {
   switch (intrinsic) {