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/code_generator_x86_64.h b/compiler/optimizing/code_generator_x86_64.h
index 39a72d8..ffc2641 100644
--- a/compiler/optimizing/code_generator_x86_64.h
+++ b/compiler/optimizing/code_generator_x86_64.h
@@ -18,13 +18,14 @@
 #define ART_COMPILER_OPTIMIZING_CODE_GENERATOR_X86_64_H_
 
 #include "arch/x86_64/instruction_set_features_x86_64.h"
+#include "base/macros.h"
 #include "code_generator.h"
 #include "driver/compiler_options.h"
 #include "nodes.h"
 #include "parallel_move_resolver.h"
 #include "utils/x86_64/assembler_x86_64.h"
 
-namespace art {
+namespace art HIDDEN {
 namespace x86_64 {
 
 // Use a local definition to prevent copying mistakes.