Add symbol versioning

Add symbol versioning to the roctracer64 and roctx64 library, and only
expose the OnLoad and OnUnload tracer_tool symbols.

Change-Id: I7f160fc3e568567fd1146ff5b9c0aef3bdcccf53

test/tool/exportmap

@@ -0,0 +1 @@
+{ global: OnLoad; OnUnload; local: *; };