Add linker version script to explicitly expose symbols. Refs #683

* When statically linking libstdc++ by default the symbols are all
  default visibility. This causes a problem if you statically link an
  old libstdc++ then inject librenderdoc.so, and the application starts
  using the old libstdc++ instead of the system's.
* Really we only want to use the static one for calls from
  librenderdoc.so. In theory -Wl,--exclude-libs,libstdc++ should do this
  for us, but it starts to cause bizarre crashes.
* Instead, we have to resort to a manual linker script which whitelists
  which symbols should be visible. Normally we can do this just for our
  own symbols with -fvisibility=hidden and __attribute__((visibility))
  but that doesn't work for linked symbols apparently.

renderdoc/renderdoc.version

@@ -0,0 +1,12 @@
+{
+        global:
+                _init;
+                _fini;
+                gl[A-Z]*;
+                dlopen;
+                _exit;
+                RENDERDOC_*;
+                VK_LAYER_RENDERDOC_*;
+        local:
+                *;
+};